Forming a D shaped hole

[Flathead_Dave]

Drill the shaft and use a castelated nut and split pin

 

[Gareth J]

Make it in two bits, the majority of the depth can be round, then just make (or buy?) A d shaped washer to go on the top. 1/4" thick would be plenty and easy to file.

 

[Maker]

Drill to size, then broach a keyway and size a key to fit against the flat?

 

[MattF]

Grub screw won't work with the clearance I have:

View attachment 62807

The spacer goes beyond the second thread.

You've lost me, between the photo and description. Unless my brain is just having a moment, your description and the photo seem to be contradicting each other. Does the spacer drop into that hole, or go from the other side?

 

[Maker]

What about getting it waterjet cut?