Given the slippery surfaces of sil-nylon, I don't think this would be a particularly good choice. You could coat the edges or the whole piece using the Jim Woods silicon sealer/mineral spirits mix. This would keep the edges from unraveling and would also cut down on sliding and increase the water proof-ness of your fabric.

Personally, I would get a piece of kite Tyvek. If you get the flexible type it only weighs 1.3 oz per square yard and does not need to have the edges finished. It isn't water proof but neither is sil-nylon. If you want light and waterproof, get a piece of 2 mil polyethylene sheet.