The premade covers are of knit so that they fit anything.
I like the sunbrella as well. Wears well, don't scratch or crock, and can be cleaned easily, and stays looking good much longer than the knits. But they pretty much have to be made to fit the fender.
To make them measure around the fender, and along the fender from center end point to center tether, NOT just the side length. You have to account for the curve ; -)
Cut fabric with an inch extra around and 3 inches extra in length. Zig Zag all around, or cut with a heat knife to seal edges.
Wrap around fender and mark the seam allowance up side, It should be around 1/2". Sew up from top to bottom.
Turn up bottom inside 1 1/2" Stitch with 1/2" allowance leaving a 1" pocket. Run shock cord or nylon cord thru pocket, draw up tight and secure with toggle or knot
Test on fender to make sure it's the right length. If it is then go to next step. If it is a bit long then turn the top edge under further. If it's a bit short then turn it under less, or let out the bottom pocket a bit.
Turn up top inside and repete. Make sure you cut the top cord long enough for the cover to be fully opened and slid over the fender before drawing it up tight and securing it.
If you know someone with a fancy sewing maching that does embroidery you can have the boat name stitched on before you sew it all up. That way if you drop a fender it may find it's way back to you!