No idea on the demand for such a thing, but wanted to throw this out there to you. When I was a young teen, I built my own display case for some knives out of a custom made box (by me) that was about 16X12X2, I laid a piece if styrofoam in it, traced the outline of my knives I wanted to put in, inletted the foam with a knife, covered it with velvet, and then glued it into the box. It turned out pretty nice. Just thought I would share.
Personally, I wouldn't put my guns on display. If someone breaks in, I want them to have to look for the good stuff and hopefully miss it.