Cutting out vacu-form parts can be a delicate operation.
Take your time!
The oldest method is probably the best -
Draw around the bottom recess with a ball point pen. This gives you a defined line to cut on.
Score (no need to cut all the way through) along the ink line with a sharp knife.
Bend back and forth along the scored line until the flat excess cracks off.
You can break off the scrap plastic in short segments. Cut lines (like pieces of a pie) away from the scored line for easier bend and separation.
Sand the bottom edges smooth with 220 grit on a block.
You are reaching the finished edge when you notice small strings of thin plastic rolling off the bottom.

For comparison,
On the left is the dome after cutting off the sheet.
On the right,
The dome after sanding the base. Sand off all the turned lip so the sides are flat.