Do what I did with my custom flattener spacer - buy a non-rotating helical focuser as the "end piece" (assuming your flattener has sufficient distance). I simply added a small T spacer to mine, then the T threaded locking helical focuser (Borg and TS sell these) and that way I could very precisely try differing distances until it was flat. Only issue is the terminator is 1.25" (fine for smaller sensors and 1.25" filters).
Then again I have never bought an off-the-shelf spacer - always had them custom machined to my design (in Brisbane Suavi, by an astro-nut like us
). I didn't use the helical in the SW Esprit 80ED, because it was right in the first place