I also carry with me an oversized tin mug (used in Central America to boil water and purchased from the market in Antigua, Guatemala for a dollar or 2) and a reusable cutlery set (so good to be able to refuse the disposable plastic forks when buying fruit salads and the like on the street) so I can literally eat my muesli anywhere.
Something that initially bothered me in this "muesli anywhere" scenario was the fact that non dairy milk isn't easy to find anywhere. In fact it's pretty darn hard to find in Latin America, and difficult to store while you're on the move every couple of days, with questionable access to refrigeration. Enter: powdered soy milk, which I found for the first time at a tiny little tienda in a tiny little beach town in Nicaragua. I guess I hadn't really thought to look for it before that, but it just added to my muesli delight. Since then I have found it in larger supermarkets in a number of cities all over Central and South America, supermarkets in these parts aren't generally as easy to come by as I'm used to, but they're there, even though I try to avoid them as I try to support the guys at the local markets, they can be handy for stuff like this. Prior to that I had been very happily getting by with adding water to my muesli, which with the benefit of the raw oats turned into a kind of milk of its own, and occasionally buying overpriced non-dairy milks when I could find them/ knew that I'd be in a place for long enough to consume it all, where I also had access to refrigeration. I was also carrying around a bag of powdered maca root that helped give the illusion of milk. I know some people like to add fruit juice to their muesli but that's never really floated my boat, it's another option to consider though. Most of the time, the milk thing never really bothered me though as my muesli was usually so full of goodness that I didn't notice. As here's the real kicker, muesli is just a really great vehicle for fruit! Sure you could just eat fruit salad for breakfast, as lord knows, Latin America is abundant in delicious tropical fruits, and when given the opportunity I most definitely did do that. Sometimes though, it's not easy to get access to all of the tropical fruits, and maybe a banana or two is all that's attainable. Bananas with muesli has been my staple breakfast on and off for many, many years now, and it still makes me so happy. As well as this, even if I am eating just fruit for breakfast sometimes, if it's going to be a big day, and I don't know how nutritious my next meal is going to be, sometimes I like to beef it up with a bit of muesli, just to make sure I get that satisfied feeling of full when I'm done eating. You can add all kinds of other stuff to muesli as well like Peanut butter, Almond butter, protein powders, whatever you like.
"So what is in this magical muesli?" I can hear you asking, settle down, I'm getting to it. Also there's no real straight answer to that, as it changes, depending on what I can get my hands on. If I find myself in a place that sells nuts, I'll do my best to stock up, prices permitting, sometimes nuts can be crazy expensive in Latin America. Basically it starts with a base of oats, always oats, unless you're gluten free, then in places like Colombia, Ecuador and Peru you'll start seeing quinoa puffs on the supermarket shelves, which is great, in Mexico, Amaranth is cheap. For me though, oats are my best friend, and actually technically gluten free, it's just the fact that they are commonly processed on the same equipment as wheat that gives them gluten.
You can start off your muesli by buying a commercial brand that is already premixed, I've found that these are usually packed full of sugar, honey or high fructose corn syrup though so I generally avoid them. Although if I think back to when my muesli started it may have been with a little premixed bag that I found at a health food shop somewhere in Mexico. Then over the following months I've added to it, it's depleted some, then I've added to it again, if I can see it's getting low, I'll add a bunch of oats, which can be pretty much found ANYWHERE, if I have just bought a bounty of nuts and seeds I'll hold on to them separately so I can keep the ratios right and make sure that I have them for the next time that I add oats, to keep some semblance of order in the chaos that is my ad hoc muesli.
Sometimes in a pinch, when I can only find pre mixed muesli and my mix is in danger of becoming mostly oats, due to lack of access to nuts and seeds, I'll buy a premixed bag to keep up the variety, but only if it's sugar free or at least low sugar. The best thing about making your own muesli is that you're the boss of it. You can add whatever you like, in the quantities that you like, I don't like dried fruit in my muesli, so I just keep it out. The following is a list of things that have made appearances, or are long time inhabitants of my little ziplock bag of goodness: Oats, Almonds, Brazil nuts, hazelnuts, cashews, peanuts, pecans, walnuts, sesame seeds, sunflower seeds, pumpkin seeds, chia seeds, hemp seeds, puffed quinoa, flaxseeds, amaranth, cacao nibs and cinnamon, always cinnamon. I'm really not a fan of dried fruit in my muesli but there's always the option to add raisins, dried cranberries or whatever you like.
Some things are cheaper in some countries than they are in others, as a rule though, oats are generally pretty darned cheap. Nuts tend to be a bit more expensive, except for peanuts, they're pretty cheap everywhere, and some seeds are cheap in most places like sesame seeds. To make the muesli you're going to want to chop the nuts up as finely as you want them, sometimes you'll have access to great knives and chopping boards, sometimes it's a blunt knife and a plastic plate. I have found ways to make do and I'm sure that you will to. When I was still in the states I was roasting my muesli as that was previously the way I liked it*. Now I find that it is very rare to have access to an oven while traveling in Latin America so I don't bother with that. Also as I mentioned earlier, when there is no access to milk adding a little water to raw oats creates its own kind of milk like liquid. This is where the cinnamon comes in, it can mask that watery flavor really well I've found. It also just gives a great flavour to anything vaguely desert like. Most of the time there's no handy mixing bowl with which to ensure that all ingredients are perfectly combined. So my method is usually one of adding it all to my chosen vessel and shaking it all about, with a little extra shake right before I serve a bowlful. My chosen vessel when I was traveling in a van was a handy Tupperware container but now that I'm back to a backpack I'm carrying it in a ziplock bag, actually I've taken to double bagging it as I had too many incidents of the single bag coming open (possibly due to changes in altitude) and finding muesli throughout my food bag (yes, I carry a food bag, and I'm not ashamed).
*If you are planning on roasting your own muesli, mixing it up with coconut oil and maple syrup or some fruit juice then baking it on a tray at 180 Celsius for a few minutes is a great way to do it.