Yet another application for one of my favorite tools, two-sided carpet tape!
Unlike masking tapes, it comes with a stiff backing, so it's easy to trim to a precise shape. You can leave the backing on - or attach a piece of styrene sheet - to make a solid edge to follow with your scribing tool. (Get the tool that's made in Pakistan. They're available from Squadron, Micro Mark and other places. Once you've used one of these, you'll never again try to scribe a panel gap with the back of a #11 Xacto blade.)
I use carpet tape a lot. Attached to a paddle, it's perfect for holding small parts while I paint them. It can also be stuck to a work surface, where it holds PE and other small parts for gluing.
Ddms