That's great! You have an actual practical reason for making one
The one I made is crochet, but if you look at the pattern page, it's simply a doily pattern but in much thicker yarn (I used Red Heart because it's so sturdy, I thought it best for outdoor use). If you only knit, just search Ravelry for a doily pattern which works from the center out. My Ravelry project page also showed how I easily attached it to my tire (elastic around the outer edges).