Pics of prototype tank below. Will be testing it over the next few weeks and months so will keep you all updated. I've already made a few tweaks to the design for the next one. This one is aimed at proving the principle.
Did this aux tank ever materialize? I too am in the market to stretch the legs of my X Ch, as I'm only getting about 100 miles per tank. I have a line on an extended XTank (10 litres) but the seller wants a lot for it, although it is still cheaper than the TT front mounted tank.