I found that with the Alpa reversing adapter (which seems to be the same dimensions as the SWA base) that the B22 is a good fit. I use that on my Alpa FPS on the SW/SWA reversing foot.
Not too big, not too small, and still keeping square and twist resistant with the lip of the plate against the Alpa foot.
Hey, what do you know ... RRS thought so too
RRS recommendation for Alpa 12-SWH
This is on the reversing foot: