Extension Rings for RZ (again)

I made a post some time back about extension rings - yet another thing I haven't yet got around to sorting - but am looking to use them for something I want to work on very soon. I'm looking for RZ rings (established RB rings won't function on the RZ) probably second-hand.

I'd value comments from anyone with experience with these - whether it's sensible to get one, or both, how useful/successful you found them in particular .

Thanks!

Sorry, put this in the wrong forum somehow but can't find any delete function, only edit. Will re-post in correct forum - mods please delete!