In your case, Michael, I would get another double U-joint. I'd cut a slit in the end that goes on the rack, and open it up just a little. Then weld some extra material and drill a through-bolt clamp that will line up exactly with the flat.
I'm not sure your set-screw arrangement is good enough, unless you drilled a deep countersink for the grub screw in the rack shaft.
@Gordon Nichols It isn't that crucial for the rack to be absolutely perfectly centered, if it's within an 1/8(+/- 1/16th from center) turn from center the wheel can be turned and the rest can be done with the tie-rods.