Aye beezer764,
No, I'm no talking about the "self adhesive neoprene stuff ". use good heavy, quality car upholstery vinyl with the cloth backing. Go and have a blether with an upholsterer, you may be able to buy materials from them or pick up a few tips afore doing it yourself.
If you are going to use wood beneath the seat base, you must check your clearances, either bond the wood to the base using more fibreglass or bolt it, but remember vibration and fibreglass, don't mix! Make sure the wood is treated or else you'll end up siting on a Morris Traveller window box!

Use newspaper from which to create your cutting templates and allow for expansion and contraction of the cloth and foam so that the stitches don't burst the fabric.
My regards, Bill.