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.