Spent hours fabricating a mold for the fiberglass vents. Used black PVC unions for 3 1/2" pipe. These have a 4" OD. Fashioned a means to turn these in the drill press to sand. Used progressively finer paper starting with 120 grit and ending with 600 grit wet/dry. Followed this up with 4 coats of wax for a glossy smooth finish. The fiberglass won't stick to this mold!
Mold in action rolling on a 5" x 50" long strip of pre-wetted 8oz cloth