by 42swing » Thu Apr 16, 2020 12:31 pm
Most such parts are what are referred to as vacuformed, rather than injection molded. Heated plastic sheets are drawn down over a mold using a vacuum.
Don't try to cut the part out directly, using a sharp Xacto knife trim around the part leaving an 1/8 in. or so as a flange around the part. Once separated, sand the back of the part on a piece of medium grit w/d sandpaper attached to a flat board in a circular motion using light and even pressure. What you're trying to do is to slowly reduce the thickness of the flange until it can be peeled away. Check for fit, you may need to remove a little more, but not usually.
To help with assembly, you might want to glue some strips of leftover plastic sheet to one half to help align the parts and provide a larger gluing surface.