Love your cover!! Great job.
i had an old cover that I recently replaced (not as ingenious as you), and when I took it off, I was surprised that I had never noticed it had a ribbon sewn on one side, and a loop on the other. The ribbon ran through the loop, with a piece of velcro attached at the appropriate places, to hold the cover taunt. Sounds like you found a fix, but if it doesn't work, you might try that little trick.