Harbor Freight among others has self fusing silicone tape. Here is a link
http://www.harborfreight.com/1-in-x-...ape-68401.html
You could cut off a strip the size of the doughnut you need, stretch and wind it on, trim excess and in about 30 minutes it will self fuse.

One could also use the liquid black tape, it is much messier and will have to dry but would make a nice durable ball

Rod