I also made one for my last boat.
A few suggestions or variations.

In Walmart, camping section you can buy a sleeping mat of closed cell foam 3/8" thick (about $13 - maybe less) which makes a great padding material for a back band.

for fabric, I used Sunbrella (used for boat tops - black) which a boat top place gave me a big scrap for nothing.

I contacted cemented my foam on to the plastic . My strap weaves in and out of slots cut into the plastic and is contacted cemented between the plastic and the foam. It can't move or slip - no rivets needed.

I also contacted cemented the black covering cloth to the assembly which makes for no sewing and keeps it tight.

Juat a few other suggestions. The bottom line is to make it with as few purchases as necessary.