Over the last few days cut out the hardpoints for the fuel drains, Left/Right Strake and Sump, from 1/4" 2024 T3, 1" square. Sanded them smooth, drilled out to 11/32" and tapped 1/8" NPT.
Make a stepped slopey bottom for the tanks from layers of 1/4" sheet. [Timewarp: Occured to me to test the urethane glue I have been using to join sheets of foam. Fuel attacks it, and my test piece failed at the glueline. When the fuel has dried off the strength returns, but a quick test piece of 5-min epoxy shows no reaction to fuel. All joints to be done with micro or 5-min epoxy.]
Do some dodgy math, and figure the sump tank will hold about 6.5 litres.