Bomar has the plastic!
Tom: Bomar has the replacement plastic, or you can go to a local plastics shop. To remove the existing piece, you can dig a hole in the caulking around the plastic and run a piece of 90# fishing line thru the hole. Then wrap a piece of wood on each end of the fishing line and just pull it around the frame. This will cut the caulking and release the existing piece. They will also be able to recommend the correct caulking to rebed the new plastic. Be sure that you mask everything very good before you start the rebedding process. Once it cures you can use a single edge razor blade to clean up the excess caulk.
