I used the cheapest shadow boxes I could find at the local craft store and lined the back with black felt. I can fit 2 decks per box and they turned out pretty good. I have been mass producing them for myself and this was the set up I had a few years ago. The only problem that I have found is trying to display decks when there are 3 in a group.
I think currently I have about 200 of the frames ready for decks so can hold about 400 of the decks in the collection.