This could be a place to start looking.
HiwayI just don't know if the size you are asking for is anything but custom.
With all the cost of doing an alloy box, would it be fitting to just buy a certified Manchester RV tank and mount it crosswise between your taillights? New no hassle fill and the right capacity!
If going the box...then think about the weight the back wall must support...that is why I suggested a tall box standing on bumper. yes you'd have to mod you wood carrier or move it over...with a sidedoor there are loads of solutions. It will be interesting to see the results.
