On roundball diameter, if I were you I'd start with what I had!
Do you have any roundball moulds in the neighborhood of your bore to groove diameters?

If not, I'd pull up the Track of The Wolf website (or just give them a call...) and see what they have - then choose something that looks right with a good thick (.020++") denim patch. That's been working out very well for Daryl (page through his list of postings via his profile here) and I'd just follow his steps for a 'quick and dirty' path to bliss!

Cheers
Tinker