David,
I don't know anything about fabrics that are to be used on kayaks or exactly how to temporarily attach it to a S&G hull, but the prior posts are excelent ideas. When you built the hull, did you use the "deck side" panel from the plans or did you just attach the bottom to the side panels? If you do use the deck side, you can attach a shearclamp along the top of this panel and place deckbeams between the sheers as often as needed for your deck design. The deckbeams could be made from any type of lightweight wood cut from 3/4" or even thinner boards or laminate thin strips clamped over a curved form. The strength of these deckbeams is probably more important with the fabric cover than it is with the permanent wood/fiberglass deck. I hope that made sense.
This picture is a small sized S&G Guilemot with the "deck side" panel.
SMF