I keep most of my first aid kits in freezer Ziplocs. For my big kit, I use the pint or quarts to make different modules, then it all packs into a gallon bag. Well, now it's expanded to two bags, but you get the idea.

If I needed something sturdier, I'd stuff the ziploc into a camera case, since I seem to collect those in various sizes. They (and the different media cases, etc.) can be very cheap on clearance sales.