You can buy pre-fab boxes ("truck boxes") or make your own.
We did our propane tank storage like this
. I tested it by scrunching my 190-lb self partially into the box -- no, I didn't fit
but the box did take all of my weight without protest.
I think wood box construction with metal frame/support/bracing works. Others go with sheet metal box/metal frame.
You may be able to find truck boxes used or at a salvage yard.
Best of luck--