I made my racks by drilling over lapping 1" holes side by side. Kind of a flat figure eight. To get the gun into the space you have to twist it in sideways slightly. I lined them with a soft rubber/silicone mesh so the barrels did not get rubbed. By keeping them with a small potion overlapping the barrels the racks firmly hold the guns in place so that even if I tip the safe they can not fall out of the racks.
Don't know if you can get your racks out but I would try making them in a soft wood, which I would then fit into the safe. If you are looking for maximum capacity you could alternate putting one gun barrels down and one barrels up. Then add a soft strap to the top rack to keep the butts firmly in place. Or even set one side of the rack for barrels and one for butts on the middle rack. Drill a deep set of two to three one inch holes to fit the stock into on the back side of your rack. Small elastic or light bungee cord holds the butt in the rack. It might nit seem like a big improvement but I can get an extra eight or ten guns stored the what is a otherwise full safe area.
