Like these They're great for getting flies home, but I frequently fill em up with flies and throw them in my chest pack, or even stick one in my pocket for those ultralight trips to a nearby pond. Some shops even have divided ones that are handy for small flies.
I do the same as patrick. They fit nicely in my chestpack. I have one that has well over two hundred chironomids in it.
I use old Red's containers almost more than any other box. I fill them up for specific trip; I may take only one for a quick trip to a creek while I may have 6 or more for a weekend trip, in addition to 'real' boxes.