What Mr. Tilley said.
Take a rectangular piece of hypalon pi times diameter of your scuppers (plus a bit for a seam) by however long you want them, glue it up, and hose clamp it on (or glue it, if you wish.)
4 inch diameter scuppers would yield a circumference of 12.56", add an inch or so for the overlap at the seam, and make it as long as you need in the other direction.
jky
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