I put it in and cut to about the right measurment but a shade longer........ Epoxy it and when it is about 3/4 cured go back and trim it to fit.
A slight push with a finger on it will make the cut ends stick after it is trimmed since the epoxy is still tackey.
I do not go over the ribs .... only a 1/4 inch up on each side. When you go back and put the final coat of epoxy on there it covers everything , including the exposed part of the ribs. They should of already been epoxy saturated before this process........
If you wanted to cover the top of the ribs then use some fiberglass tape.
Chuck.