Getting back to the subject of the taillight boards.
This plan has fallen through, so I'm again looking at cutting them by hand.
Unfortunately I'm not able to make my cuts accurate enough using my pen knife. To cut through the material I need to make several passes, and the smaller cuts waver because of it. I'm looking at getting a heated knife, because not only would it make replacing the rear quarter glass easier (see other posts), but it also might cut Delrin better (and, I hope, faster) than several passes of an Xacto blade.