perhaps not ideal, but convert a condemned LPG tank?
The bigger si2es are popular with inboard ski boats, so I'm told.
One of those skinny ones made to go as a set with others into utes and panel vans?
Adapting the filler could be a problem, if you are looking to use it as the sole petrol tank, but I can imagine the standard plumbing si2es would cope with a sub tank pump filling it