Looking for such an eyepiece for my 50mm Stellarvue RACI finder.
Those I'm considering...
BST Starguider (and clones)
Lunt Solar Flat Field
BST Flat Field
Celestron X-Cel LX
ES52 10mm (yeah, I know, not 12mm and not 60º but maybe)
What I want is the best choice that's likely to get closest to an aberration free field.