I think I've found a good solution, magnets. These are very strong magnets for their size, and they are rubber coated, so the guns & the safe will not get nicked up.
I tried a round magnet, but it allowed the gun to pivot even though it was plenty strong enough (32.6 Lb of strength) to hold the gun. I ended up using two rectangular magnets per gun, a single magnet had enough strength (8.26 LB) to hold the gun up without sliding, but the single magnet was canted at a angle I wasn't comfortable with. I used the pair of magnets end to end, and they held the gun firmly with no issues.
I used a Freedom Arms model 83 in the photos, but I found 2 magnets was also better with a 1911, (gun still canted)
I tried a pair of plastic coated magnets as well, but the slicker plastic allowed the gun to slide down the door.
The magnets were also able to hold with a piece of cardboard stuck to the door of the safe, so they'd probably hold on carpeting. If not a third magnet added in would definitely hold on carpeting.
For anyone interested in trying this for themselves I bought the magnets from K&J Magnetics. The part number is BX084BR-N52. These magnets are $5 each., and they're "Rare Earth" magnets in grade N52 (strength). You can buy your own rare earth magnets & coat them with plasti-dip, but if you have to buy everything any way, I doubt you'd save much.