Now slide the Y axis C beams onto the structure:
And let it rest on the 2040 rails that look 5 mm shorter:
And attach the corner plates with 15mm screws to the C-beams and with 12mm screws t-nuts in the 2040 rails:
At the backside of the machine, also add the 2 stepper motors for the Y axis, and attach the leadscrews to them with flexible couplers: