I got this idea from a buddy of mine:
For years I kept all my guns in a closet that I modified into a semi secure storage space. I reinforced the studs around the door by screwing another 2x4 to them.
Then, I replaced the short hinge and lockplate screws with longer screws.
After that, I replaced the interior door knob with a dead bolt.
Finally, I built a shelf around the closet about 18" above the floor and attached a piece of 2x4to the wall about 25" above the shelf.
I cut 1/4" surgical tubing into pieces 2" long and screwed 3" sheetrock screws through it into the 2x4 about every 3".
I ended up with a fairly secure space that had a rack on three sides for my long guns, space underneath the shelf for .50 cal ammo cans stacked 2 high, a shelf on top for handguns and accessories, and I used the clothes bar to hang LBVs and camouflage clothing.
It wasn't as secure as the $1200 safes I have now, but it was big enough to hold about 50 long guns and 10's of thousands of rounds of ammo. At the time I had my C&R FFL and was buying $40 mosins and mausers, $100 SKSs, and cases of ammo at 6 or 7 cents/rd.
My entire collection cost less than the price of one good safe and those guns and ammo appreciated by 4 or 5 hundred percent. The safe would be worth about the same, so I think it was a good strategy.
It looked like this:
