I did that once upon a time. I used 1/4" last-a-foam to build my tank. It was a square when viewed from above and a rectangle when viewed from the rear. From the side view, it was a trapazoid with the wedge portion in front to go up against the seat back. It needs a 3/4" aluminum fuel line along the side that will fill and drain the tank. Put a 1/4" aluminum tube out the top to go to a plastic tubing vent. Start by cutting the foam pieces of the box and glue them together with 5 minute glue to form an open top box. (Don't put the top on yet, duh) Put in the 3/4" fuel tube in either forward corner (your choice) and make it about level with the bottom and as close to the bottom as you can. This tube will extend forward into the the main tank. Micro the inside of the box, flox the corners and around the fuel tube and lay up two real wet layers of bid. Do the inside of the top piece too. Don't forget to put the vent tube in the top piece. When the whole mess is pretty stiff but not fully cured, stick the top on with plenty of wet flox. When cured on the inside, pressure test using th balloon method. If it's OK, round the foam box corners and glass with two layers of bid over the exterior. The finished tank is slid in from the rear with a hole cut through the seat back and main tank to allow the fuel tube into the main tank. Flox the fuel tube into the main tank and fix the aux tank in the back with glassed foam tabs and run a vent tube somewhere high and outside. Make sure the tank is pinned down tight (maybe a sheet of rubber under it) or it will rub due to vibration and damage itself. The reason for the large fuel line is because of the Rotax's famous high fuel burn. (just kidding) You can fill the main tank on the ground and it will fill the aux tank through the large fuel tube so you don't have to build a new filler port for the aux tank. In flight attitude, the aux tank drains into the main tank. By the way, the tank was about 5" tall on the sides and held about 2.6 gallons.