http://42draftdesigns.com makes exhaust tips and will modify any measurement on the tip to meet your exact need.
Also, I would never powdercoat exhaust tips as it cannot withstand the heat. The inside of the tips will bubble up and look terrible. Contact JetHot (
http://www.jet-hot.com/) in Oklahoma to have them JetHot coated. The outcome is amazing and they always look new.
Brian