I thought I would post my experience trying to cut a hole in a plastic box. This box was a makeup box with a metal frame and plastic sides. I ruined three of them trying different methods to cut the plastic. I tried drilling multiple holes; I tried using a cutting knife; I tried using a jigsaw. With each method I cracked the plastic. Then it hit me - I could melt the plastic with a woodburning kit (or sodering iron). I did this and the fourth time is a charm. FYI - If you ever need to cut a hole in plastic, and want a smooth edge - consider melting the material.

If anyone else has experience to share about cutting plastic, please add your thoughts...

I use a soldering iron

Be careful of the fumes. If you do it infrequently, it may only give you a raging headache, but some types of plastic can emit rather toxic fumes when melted.

-Erik

Of course it depends on the plastic, I am mostly familiar with styrene as that's what we use in model and prop building a lot.

Scribing is a very good way. With a ruler as a guide you scribe the plastic bit at a time-you don't have to cut all the way thru as you can snap it along the scribed seam easily. You could use a screwdriver as a scriber, though there are spesific tools for that.

For round holes, I mostly use a handy tool called Circlescribe, which you can check out here:

http://www.circlescribe.co.uk/First%20page.htm

It is very handy on styrene, I often work with 1/32, 1/16+ and 3/32+ thick styrene. Same method here, scribing bit by bit by bit.