Okay here's what I did:
side view of the crossbar:
endplate:
The crossbar is a 6mm square aluminum rod. One cool thing about aluminum - it is soft enough that you don't need to tap the screw holes, the brass M4 screws I used formed good threads simply by screwing them into 4 mm holes. The endplates were made from some roof flashing I had laying around. I'd recommend some variety of steel, anything from 0.5 mm (24 gauge) to 1.0 mm ( 18 gauge).
The lug holes were rough drilled, then finished with a file. Working on 2 or more at once (clamp a stack of them together) makes life easier. Decide how long to make the endplate tangs based on your block - it is 7.5 to 8 mm from lug bottom to die top. The 90 degree bend in the endplates where they wrap over the crossbar must be perfectly parallel to the bottom!
I also drilled a small dimple in the center of the top of the block, to give positive placement of the thrust screw.