The old standard X-frame............
rack, I'm sure you've seen them outside at ranges. Instead of the little radiussed cuts at the top for barrels, put wider and DEEPER cuts for buttstocks. You can even angle them slightly all one way, to avoid the toe of the stock getting caught by an unwary passerby. I didn't have the money to construct a vault w/overhead facilities for hanging, so, I just put them in snug Caplugs, which can be used as inners or outers, over rifle barrels, and IN shotgun barrels. I wanted to design sides, rears, and islands for safes, but am really challenged right now, so haven't had the time,.......y'know how that is, right?? Just make sure the rifles have enough rake in the angle they're set to keep them back, and enough of a set-out on the bottom where the muzzles are resting, and make sure there is plenty of room up and down, behind to clear any 'scopes. A spade drill can make some small depressions where the Caplugs can drop in, maybe 1/4" deep should be enough, I guess.
Oh, and the Caplugs can be wiped out and re-used until they start to get frayed, or worn bad. I haven't seen any ill effects from heavy-barrelled varmint guns stored "at a lean" yet. Another thought just occurred, one of those Ft. Sandflat carousels custom-built w/a larger diameter top and bottom should work, also, the one that Wal-Mart used to use; I always thought that had possibilities..... Hope This Helps