Cutting a ceiling frothy cover. The rugged brittle plastic type. It keep shattering when I cut it to size? Any tips?

Any tips?
Why do buildings surrounded by UK enjoy bricks...
Answers:    You're right. This isn't easy! Ensure the plastic is thaw out (above 70 degrees). Scoring over and over with a utility stick is safest though tedious. A cup cutter with straight lip and snapping it off should work also.
Rusted Water?
use a decoration saw, or a elect plastic cutter, or i just use my soldering gun,