Will a Dremel multipurpose bit cut through 1/2 inch polycarbonate?
Polycarbonate likes to melt when you hit it with a high speed blade. The teeth on your 1/8" spiral cutting bit will quickly goo up, and you'll find yourself melting rather than sawing. With some skill/practice, you'll probably be able to make your plastic into about the shape you want. Biggest trouble will probably come from you making the cut too quickly, which'll cause the bit to flex, and melt an irregular path top-to bottom through your Lexan.

-I'd use a jig saw. It's a lot of work, but far less prone to disaster than 30,000 rpm cutting bits.