Pics used for illustration only.
Notes:
1. The O-ring shown is just to save me losing the collets when not in use....
2. I have long ago forgotten how to draw in CAD, so here are the collet machining details:
Machine the pair in 1 piece from bar stock 1045 or better- I used a bit of 4140. No heat treatment.
Collet body diameter 0.812" (20.62mm) x 24mm long.
Flange diameter 25.5mm x 2.2mm wide.
Tap M12 right through.
Slot 5mm wide end to end, to make 2 collets. I used a slot drill in the mill, held in vice, did 1 side then turned it over.
Deburr edges, hold the pair of cut collets in a 4 jaw chuck and clock up the oval flange so it's concentric.
Turn the flange to 0.805" diameter, so that it will fit through the bearing without the stud in.
Stud is 12mm threaded rod, 120mm long.
To use, slip the collets into the bearing, poke a bar in the back to check they are level with each other, and screw the stud in. Then put a 1-1/16" 1/2" drive socket over it, fit washer and tighten nut. You could make up an outer body instead, but this works fine.