I have made my own version of the portlight covers - not as elegant as the ones Seaworthy used to make, but they seem to do the job.
The plastic is 2mm vinyl, the same material used in the hanging door strips in industrial freezers. It was a few $/ft.
Basically I wanted to go around half the circumference of each size portlight, and I only wanted it to stick out 3 inches from the wall. I tried just cutting 3" rectangular strips, but it didn't look very good.
The shape of the vinyl is an arc, not a full half circle, 13" long and 3" tall for the 8" portlights. Cutting that exact curve was the tricky part. And finding the right tool to do it.
I found trammel points at a tool store and mickey-moused a set-up to cut the vinyl. I think it worked well and looks ok. Next time I'd use a cutting board instead of cardboard - the knife sunk into it, slowing it down.
The vinyl is pliable enough that it won't pop out of the hose clamp if you hit it (and it won't hurt a bare shin either), and rigid enough to hold it's shape as a cover. They kind of stick up at an angle, instead of covering half the portlight like the seaworthy ones do. In a real blow with sideways rain they won't do much, but for the average downpour they should be fine.