I have made feminine hygiene kits using the specifications found on daysforgirls.com. These are washable menstrual pads, and the shield snaps under the crotch of panties. The national organization has been doing these kits for years and has modified their specifications based on feedback over time. They are convinced these snaps are the best thing for this purpose. The shields that have the snaps are expected to last for 3 years, and have been found to do just that.
The snap system that they recommend is called Kamsnaps.com. Snaps are plastic and come in more than one size. The kit that I bought had lots of snaps in multiple colors plus the pliers type tool.
I am not sure that this would work for your purpose, but since my work on that project, I have used the leftover snaps on other things. It is possible that plastic snaps would eventually warp or crack, but if, as you say, the bag would not be taken on and off very often, I think they might be fine. One advantage to the box of multiple snaps is that they come in a variety of colors.
I purchased mine from the website, but I have seen them on Ebay also.
Best wishes with your project. Those bags are not only useful, but they make the recipient feel special and often become the starting point for conversation.