Use a painter's drop cloth to make your pattern...
They are cheap at home store. Hang it over your pedastal/wheel and fit it by making your pleats/seams on the outside with pins. Cut your zipper seam last - so you can leave your pinned pleats in place and this will ensure that you can get it on/off after it's sewn. Cut it up, and there's your pattern - transfer it to sunbrella and sew it up.
LaLeLu 40' Caliber
Merritt Island, FL