This is going to be long. Time me if you want. Start:
Drill with M4 bit and a vacuum cleaner at this point to avoid getting chips in unwanted places and shortening something.
Assembled an L shape bracket with a bolt like so:
Cut 2 x M4 threaded rod pieces in 125mm lengths:
Assemble the rods and L shape into:
Bend the rods to be perpendicular to the Meccano piece:
Now attach the Meccano piece with some M5 Inbus:
I had to recenter my X- axis to be in the center of the rods.
Finally bolt together, tighten and adjust with top lock nuts.
Phew! That was a fast one. Took me longer to stop and take photos, than to actually assemble.