I've got a new 3d printer on the way that may be just the ticket for drop tanks. Not quite big enough to do them all in one go but I could probably do them in 3 pieces. They could be made to key together for a strong and precise joint that would just need a little filler to be perfect. The printer uses resin and is high resolution so the finished surface is nearly perfect. I did a plug for a vacuum formed 1/6th scale drop tank some years back but it was just too large for my forming equipment. I sent the plug to a vac-forming service that offered them for a couple of months and then went out of business. Never did get my plug back!