A trick for cutting slits in any material.
Do a rough cut to make an opening. Sometimes I just drill a bunch of holes. Then use a file to slowly work the opening until you get it where you want it.
There are scissors out there specifically for cutting plastic too. You should be able to pick them up at your local hobby shop or craft store.
They are cheap and worth every penny. I like the curved version.
[URL unfurl="true"]https://www.amazon.com/Hobbypark-Trimming-Scissors-Bodyshell-Helicopter/dp/B088WVN6M5[/URL]