I'd recommend doing the superglue stitches I mentioned when you sew on the patch. It will give you some minimal waterproofing around the edges and keep things from tearing as easily when under tension. Even though my Suzuki cushion was cheapo open cell foam it never got saturated when riding it here in Seattle and when I lived in the Smoky Mts. (typically a bit more rain there than here)
If my big butt can get by with that for years, I bet you would have minimal issues for one season.