Since lowering the bathtub, the original vent cover isn’t long enough so I decided to make a new one out of the left over fiberglass I have. I made the form out of birch plywood, applied a mold release, two coats of gel-coat and laid up two layers of fiberglass mat leaving the excess hanging over the edges.
Trimmed the excess off with my table saw and miter box. I had to tear apart the form to release the part from the mold. Oh well, I didn’t need it anymore. The new cover next to the old plastic one. All that’s left is to cut it to length and fit it to the curved ceiling.