Hey Rob,
Would you recommend using a jigsaw or a dremel?? 
Hey Jay,
The last shield I made was from Lexan. I used a jigsaw for the rough cut, and a Dremel for the clean up and detail cuts. I actually used a Dremel bit that looks like a regular drill bit, but is made for side cutting (sides of the bit are the cutting edge). That worked great.
For Acrylic, I would probably just use the Dremel. It still requires going slow and steady - i.e. don't force the blade. Also, make sure the plastic is nice and warm. How much are you planning to cut off? If you're removing enough, you could make a couple of smaller practice cuts.
Any flapping or flexing on either side of the cut can cause it to crack, starting at the cut point, then off in some direction you don't want it to go. You want to be sure to support both sides after the blade cuts through, so the part you are removing isn't flapping around. You may also want to cut off the strip you are removing several times before it gets too long. The more support you have closest to the blade is best.
Using some painters masking tape will help prevent tools from scratching the plastic, but I have seen a jig saw scrape through the tape, and still leave marks. You might want to put down a couple layers of tape if you go that route.
If you use a variable speed jig saw, you should go slow with a medium or fine tooth blade. Heat from the blade will start melting the plastic, so you can get a feel for how much speed it takes before it starts overheating. Once it starts sticking to the blade, it will flex the plastic abruptly, then it's all over. It's real easy to start cutting, discover the blade is going through the plastic like a hot knife through butter, and start pushing it because it seems so easy.
