generic approx 2x photo relay lenses

These typically have 23.2mm tube to fit microscopes and C-mount thread at their opposite end.
Similar 0.5x photo relays are used with a C-mount USB camera cube:

Since Nikon CF objectives are nominally free of chromatic aberrations,
truly generic photo relay camera adapters could work well.
Brooke Clarke compared a pair of generic camera adapters

which are no longer available from cited vendors Image Solutions and Optexcom.
A relay lens acquired 20 or so years ago looks similar to that Optexcom adapter:

@ 5cm from C-mount thread to camera sensor, that image is poor and distorted,
because it is actually intended for use with a C-mount camera sensor in very close proximity.

The Image Solutions adapter looks similar to currently-available AmScope CA-CAN-SLR-III.

Its Amazon price is higher than that for a used CF PL2.5X,
but includes mechanical camera attachment needed for CF PL2.5X.