Here's a cup holder (and change tray) for 1987-1993 Mustangs. The cupholder slides down into the hole left by removing the ashtray. Tested driving with a full cup of water and it never fell out but it is a bit tougher to shift. However shifting for me was unaffected when the cup was removed which is fine for me since I don't normally drive around with cup in the car (a smaller cup might have been fine too).
Printed the upper section with supports, the other parts didn't need them. Simply glue the parts together and paint to match the interior.
cup_holder_center_section.stl | 2.2KB | |
cup_holder_lower_section.stl | 2.6KB | |
cup_holder_upper_section_rev2.stl | 349.9KB |