I may not be an expert on fiberglass but I did stay at. Holliday inn last night and this sounds like good adviceFC- you nailed it.
On a vertical surface like that you will never get a glass surface. You need to manufacture the piece and then install it.
I use a plate of tempered glass and a quick coat of wax. This will allow the resin to saturate the cloth and become the "mold". then just pop it off the glass and cut to fit and epoxy it to the bulkhead.
If you sand it after it has hardened you risk hitting the fibers and you will F it up. Make sure that you use enough resin so you don't end up with dry spots when you pop it off the glass.
I use glass because some resins don't like formica or other plastics. :-/
Casa - where do you live?