The scrap pile grows....

The most recent attempt was so close.  As usual, got wrecked by the final tool change.  Not sure what happened (maybe I just forgot to touch off?), but the final tool, which finished the linear guide grooves, was about 1mm too deep.  

This op is 1/6 of the whole part.  Eventually 3 of these, machined on both sides, will get joined by a tie rod.

3mm drill shank in the 3mm slot.  The walls are actually parallel now.

Face spline for locating the 3 pieces and transmitting torque: