I did exactly what you're planning about 8-9 years ago. I was given a big (5200# I was told) Mosler Safe that was once the business safe for a Chevrolet dealership here. I had to hire a roll back car hauler to move it for me.
Anyway, I stripped all the old wooden shelves and pigeon holes out of it an cleaned the interior good, then cut a tight nap carpet and glued it to all interior surfaces with 3M spray adhesive. Before doing so I drilled a 3/8" hole through the floor, in a corner, with an extra long HSS drill bit. It went through the inner floor, insulating material (it looked like a mixture of concrete and asbestos?) and the outside floor with ease, probably about 9 inches total thickness. I ran the cord for a large Goldenrod through the hole and put the plug on the end outside. I didn't bother building any racks or shelves, not wanting to take up any space. I just lean the guns up against the walls and haven't had any problem. I find I can squeeze more in there by not having any racks or shelves. You may rather have them, however.
The carpet has worked great and everything stays warm, dry and toasty from the heat off the Goldenrod. This is Georgia, and extremely humid, commonly running in the mid 90% level all summer. You could use the 3M adhesive, or some other construction adhesive like Liquid Nails to put paneling in it, but I prefer the carpet.
Good luck, Stan
Stan, Mine is a Mosler as well. Thanks very much for passing on your experience. Sounds like with the wood, light bulb and goldenrod, I should be good to go.
Will be creating racks as I'm essentially clumsy and need all the help I can get not banging the guns into each other. LOL
