Making non-circular holes is the part about doing this kind of work I dislike the most.
I will use a ruler and with pencil I can easily clean off, draw the outline of what the final shape is. Then I'll drill holes to take out the bulk of it, possibly cut a little away with dykes, and use metal files to slowly get it to the final shape. I have never had luck using a dremel for cutting plastic or metal; I always ended up disappointed with how it looked when I was done.
Didn't think about a file, harbor freight sells a "usable set for cheap.. I'm gunna try that! I'm good with metal, but I can't weld plastic and re finish it lol, but I don't want a metal enclosure. I'll give it a shot and post pics!
Use a "diamond" file set. They work good on plastics and metals.
Mine are just regular files and they work pretty good. Most important thing is to be patient.
Idk if I can afford diamond files right now lol. I'll be patient with cheap shit.