I fabricated the left side under body aero panel today. Created a cardboard template, then cut and shaped the panel from a 3 mm 4'x8' ABS sheet. I've never worked with ABS before, so it was a bit of a learning curve. I haven't attached it yet, but that will involve rivet nuts and stand-offs as appropriate.
FWIW high temp here in KC today was 5F. While I managed to get the workshop up to 50F with my kerosene heater, heating the ABS to bend it was still a time consuming process.