[DonRotolo]

You can purchase circles of tempered glass in various diameters. I recently bought a 4' diameter one at Pier 1 Imports for just over $100 - a three foot diameter version is somewhat less costly, and sounds like the right size for your tire. Tempered glass is very tough, hard to break, but turns into little cubes if you manage to shatter it - safe

To mount it, I'd suggest dabs of a strong adhesive (like silicone sealer) or something stronger mounted to a few metal studs, which mount through the tire.

Take a photo when it's done!