I bought a Stellarvue SFF7-2 flattener and tried it on my WO Megrez 90 with the Nikon D90 DSLR.
I have added a crop from the extreme top left region of the frame, which is where I notice the problem the most. The images were taken a week or two apart so there is a little bit of rotation present.
The SFF7-2 seems to do a pretty good job straight out of the box with no extra spacers fitted, didn't quite flatten everything right to the extreme corners of the image, but a very good result for the money.