There are macro rings for RF mount, on ebay, from who knows what manufacturer. The may be fine, but like Wadizzle says, I would be more inclined to trust tried-and-tested EF mount tubes, which have been around long enough for plenty of people to have reviewed them. Macro rings don't have to do a lot, basically just the mechanics and straight-through wiring for the electronic connection. But cheapo third-party junk can always find some way to screw up.