I don't do much Canada now that I'm DFW based and senior enough to avoid the fly through Canada stuff. But when I wasn't, I just left the fresh food at home (or crew room fridge) and put frozen dinners not for that specific night in crew room freezer and when asked in Canada what I had, I said "frozen dinner". I don't know that it was OK but I wasn't lying and they never bothered to look that I had my own frozen dinner and not store bought.

Because I didn't have anything on the return, I didn't have to worry about US customs.

I don't bring food to Mexico. I LOVE the food there and it's cheap as hell. Probably not as healthy as what I'd normally bring but it's good and cheap enough I don't care.