A friend of mine owns an Infinity FX35 (luxury suv thing) and the cup holder flap, insert, broke. Infinity wants you to buy the whole assembly for hundreds of dollars.
Yadda Yadda Yadda...a few beers, over use of calipers, and some cursing and we have a functional part.
Amazon PETG or ABS
PETG or ABS so it won't deform in the heat of a car. The hardest part is keeping the pins for the spring from breaking.