P.G. .570, Jelly Head .570, Carlson's .570 all shoot the Hevi 6's well in the two 20 gauge 870's I've built in the last 2 years. Sumtoy is another FANTASTIC option, all be it a tad higher priced...what you get from William though is a guaranteed happy customer experience as he won't be happy until you are...
He has quickly EARNED his reputation on this forum and others as the guy totalk to for quality custom chokes that won't drain your bank account.