Re: Firearm storage alternatives
Stack-on also makes a smaller cabinet that is for storing handguns & stuff. This is assuming that you have enough room for all of the long guns in your current gun locker.
Not the most secure thing, but maybe enough to make any thieves look for something easier to steal.
As for rust & cleanliness, I take out the guns I rarely shoot a couple times per year and give them a good wipe with oil and run an oiled patch down the bore. It's pretty dry here (low humidity), so this works for me.
If you are having an issue with rust you could get a big can of silica gel or one of those rod type dehumidifiers for the locker.
I'm in the same boat as you (all fit in my locker but 1 or 2), and will get a large fire rated safe when the cash flow allows it.
I keep nature balanced - I hunt everything!