Our diffuser was missing about five chunks out of it and slightly brittle, The door seal was catching on it and trying to go forward of it and I recently read about another member replacing his, door removal etc.
Went to lowes and found a trim piece for under $4 that was about 1/8" thick, cut it to the length of the diffuser, cut the longer side off on the table saw to nearly the same as the shorter, gave it 5 or 6 coats of white spray paint, held it with a piece of junk aluminum.
Lightly filled the track with some lap sealant, using a tooth pick to work it into the grove, pipped it in place, masking tape to hold the ends, looks beautiful.
Wow looks great!