I have seen this application and the one that comes to mind, is the Savage-24, survival. It has a trapdoor type construction, on the buttstock. Oh yes, then there are the SKS's that have a push button door, on the buttplate and stores a toolkit, in the buttstock. I know it can be done and in your service, a pretty good idea. ....
Funny you should mention patch boxes. I have an older Pennsylvania flintlock with a rather ornate patchbox, when I first got this M/L, I opened up the box and found the usual spares, wrapped in cloth and under that, in the stock, was and old compass. I thought that was pretty neat !! ...
Be Safe !!!