If you look at the second thread linked above you will see that I epoxied a semi-circular aluminum piece onto the flat spot. So, I made the flat spot round with a hole for the set screw. I was concerned about the set screw moving sideways on the flat surface and the added piece prevents that.
My u-joint does not have a d-shaped socket. It has a splined socket that I drilled out. So, it is smooth inside where it fits over the rack shaft.
The set screw is in compression against the flat surface and in shear against the added aluminum piece. I suppose the aluminum piece would fail before the set screw did.