They're very effective, my dinghy is now about 8 years old, and I think is in far better condition than it would be without them...

I made mine out of Stamoid. FAR easier than Sunbrella to work with for such a project, dealing with all the cutouts for oarlocks, etc... Some also argue against Sunbrella as it will have a bit more of a tendency to hold dried salt crystals, thus increasing the possible abrasion factor. (Stamoid also stows in a much less bulky form than Sunbrella, so I use it for all my covers - the only Sunbrella on my boat is the mainsail cover) Mine is form-fitted around the ends of the stern tubes, and all around the boat to a point just below the molded rubrail, and kept snug with elastomer cord run through the bottom edge, with the addition of a couple of hooks that go to the eyes for the towing bridle up front...

Bit of a labor-intensive project, you'd probably have to pay a canvas guy a fair amount to have them made... But for a DIY project, I'd recommend these highly, especially for tenders that are gonna be taken south, and remain inflated and exposed to harsher sunlight for extended periods... Mine take only a minute or two to put on or remove, but I admit I often don't bother with them when cruising up north, especially if I'm gonna be deflating the tender and bringing it back aboard regularly... But whenever I get someplace where it's gonna be in the water for awhile, they generally go on...