Defiantly something like the k wool for fireproofing, with the final inside layer being fire board to spray glue a marine rubber backed carpet to. It is easy to work with and looks really good. My safe is in a corner in a room with no windows and bolted to the floor joists in 6 places, through the back into the studs in 6 places and through one side in 6 places with 5/8 lag bolts and huge washers inside.
Forget about racks too, they take up to much room. I put each gun in the padded gun sleeves and alternate direction, when putting them in; ie one barrel down, next one up,,ect. ect. with that set up I have almost 60 rifles in a 42 gun safe not counting tons of handguns also in padded pistol rugs, also laying on there side and alternating on the top shelf.
So in that one particular 42 gun safe I safely store almost 100 guns doing it as I did and with the padded pistol rugs and padded rifle sleeves you'll never even get a scratch and can maximize the storage room in a relatively small safe ,,much better that way. I keep my safe queens in back and the ones I use all the time up front so there not really to hard to get at.
Oh BTW, seal around any bolt holes/heads from the inside with hydraulic cement to keep the safe more fire proof should there be a fire in a conventional construction house. In a major structure fire the bolts would burn away from the wood allowing fire inside the safe, with cement around them, it won't. And by installing in a corner you'll make it much more difficult for anyone to hook on to the safe in any meaningful way or to be able to be able to pry open a door. Put the handle side towards the corner wall also so it's even harder to get any type of bar between the space on the door and frame sense the wall will be in the way when trying to pry it open.
For inside lights and dehumidifier just use rechargeable stuff, I have a under counter battery operated LED light with rechargeable batteries with Velcro to hold it to the inside wherever it gives the most light,they work great. Forget about the door pistol hanger things,,they suck, pistol rugs on a top shelve work way better and will protect the guns much better while taking up much less room. And if you want to store mags on the door, just buy the cheap 3-4 dollar mag pouches, sew Velcro to the back and stick them on the inside of the door carpet, simple. Shoe hangers work well for hi cap rifle mags too, but I like pouches much better, just grab um and go.
For you guys that live in a concrete type structure or if it's in a cellar,you can also put a block wall filled with insulation or sand, or a poured wall around the safe making it even more fire proof and more secure, making sure the wall/pening for the door stands proud of the door by a couple inches or so to make it even tougher to get a bar in the door crack to get any leverage. Doors are the weak spot on any safe so make it as hard as possible to be able to get a bar into the door gap on it.
Sure,,There's more you can do, but unless you've got the contents of Fort Knox stored in there,or have super deep pockets to buy something super special, that should be just fine for the average homeowner with out spending a fortune for security.
Stay away from electronic locks too, one of the several garage door master codes will open them right up, my bud showed me that trick when he opened >15 safes in a row at just one of the stores we were in, and they had high end Liberty's and several other expensive brands there too, not just cheap ones.
That just blew the guys mind at the store when he saw that,and he just couldn't believe it, he also lost 25 bucks by betting my friend that he couldn't open up the Liberty. OOPS!!! He did, on the first try and in about 10 seconds!!! CA-CHING $$$ for my bud,,,,
Like I said, there was about 15 20 safes on one wall and my bud opened every single one in less than 2 minutes. People just started walking out or started looking at conventional dial locks when they saw that,,,,that blew me away too!! I didn't realize the electronic locks were that bad, but obviously so after seeing that one, they are.
Hope those tips help out Coils, and those are just a few things I learned over the years about safes and installing them. I'm sure some other guys will have other suggestions too.