Unistrut you should be able to bolt something together. It is sold in home stores in the electrical dept.
Or you could get a little MIG welder and teach yourself how to fab stuff. It only takes a few projects to pay for a little welder. It looks like most of the landscape trailers around here use home built stuff.
I'd look for a local Delta truck box dealer to find a fuel tank. They sell a nice tool box fuel tank combo which would also give you some secure tool storage.