I might have the wrong mental picture from your description. But if you have a recess for the butt of the gun, is it sitting down in a hole? Use felt or something to get a friction fit. The saddle for the barrel could be made a bit deep so that the gun can not go sideways, the butt keeps it from going backwards as well. So something to keep it from moving upwards is what you would be after I guess.
Depending on what you want, short dowel rods made to slide into place would work, velcro with snaps like some of the uncle mikes adjustable holsters.
I was planning to do something similar and make two or three dowel rods, so if you are removing one gun it is not a big deal. If you use metal rod with padding you could make it lockable.
Basically I was wanting to have the gun sit like you are holding it, with the bott in a friction fit, the barrel in a padded friction fit, and a dowel rod across the top towards the back so you could not lift the gun out of the part holding the butt.
I would love pictures of the inside of the finished product as I was amazed by the other gun cabinet someone posted on this site. Matter of fact I might go look for that as it made me want a nice gun cabinet.
