I just bumped into another cool approach for trimming vacuum formed shells.
This industrial designer makes a jig at the appropriate height and contour and uses an oscillating multitool with a flush cut blade attached. Very cool.
If you're not familiar with multi-tools, they're one of the coolest power tools, and the flush cutters one of their most useful attachments:
I belive that Fein originally came up with the concept, but many manufacturers make them now.