Check out farm & ranch places, internet/amazon.
Different application, I got a monster aluminum diamond plate tool box for the ATV trailer at a farm & ranch store. Holds covers, tie downs, gas cans, chairs, &c, and bolted on the inside so I can remove it when I need to haul the tractor or mower. Thief will have to rip it off to take it. Anyway, what I wanted to add is that they had at least a couple dozen designs/sizes avalable, and at lest a couple of the smaller ones would probably do you just fine.
If you know how many firearms you have without stopping to count...
...You don't have enough guns.
If the dollar value of your firearms exceeds the value of your ammunition...
...You don't have enough ammunition.