I use an ex military underseat tank on my 110 V8csw as they fill from top , so no filler problem to overcome with second row doors , I use a pump and pipe to rear main tank, this would prevent any air lock probs with your being a tdi.
I would look at exmil sites as they were standard fit on 109 mil spec cant help with part no you will need to make a bracket , very simple piece of angle with flat plate welded on one end and then triangulated from top of flat plate to end of angle to brace it then weld whole assy to chassis and drill 3 holes for front mount of tank rear uses existing chassis bracket . Then run fuel line from front tank thru facet fuel pump to T piece in return line from engine . You can either fit extra fuel guage or just wait untill rear tank is low eneough to take entire contents of front tank and turn on pump . You will know when front tank empty due to noise made by the transfer pump. HTSH