Baader makes some in 7.5mm, 15mm, and 40mm as well:
http://www.baader-planetarium.de/sek...s08.htm#+25abc
(see
http://www.optcorp.com/productList.a...id=105-182-967 for some english).
Somewhere around here I'm sure I have a few of those, and could try them with the Flat3 on the Megrez 90 if I can dig them up. I purchased them when trying, like you, to get the M90 working better with the Flat3. I'll let you know if I can find them. I seem to recall the 7.5mm being within 1mm of optimal for coupling the Flat3 to a Canon EF, but it has been a while so I can't confirm that.
Anything adjustable will unfortunately likely introduce flexure, which would be counter-productive. I used to have a variable Meade adapter, however it was way too big for the spacing you are looking for.
Regards,
Eric