Although it is not a shoulder-strap version, my carpenter's toolbag is what I transport MF gear in. One of the principal virtues is that is has outside pockets, where I stuff a toothbrush and toothpast, a pair of socks, deodorant, etc. It looks like gym-workout gear, which (in this country) would probably not be considered worth stealing.

I think that if I were going to Peru, I'd try to find some pictures of locals from the appropriate age, gender, and social classes, and see what they tend to carry around with them. A well-used Jansport would do it here (look around on BART sometime) but I don't know what the Peruvian equivalent might be.