Making the rear window sills flat?
 

Any ideas how to make the rear sills flat? Piece of plywood and some rubber? Any ideas appreciated. Don't care about cutting or drilling anything.

Section out plastic and make a sill.
 

You could section out the plastic below the window. An oscillating tool would work nicely. Depending on the thickness of the sill material you could section out a pocket and slide it in. You could also make an entire side panel and work it in

X2 ^^^ small saw as mentioned

The oscillating cutting tools are great. Clean cuts almost anywhere with the available variety of blades.

Man up. Use a saws-all. LOL

Every Man should have a sawzall!!! Where did you want that door???