I went shopping for parts yesterday, and returned with a base plate which was a bit larger (but much cheaper) than I had intended. As I can't use a circular saw in my apartment, I just updated the design to use a larger base (more feeders, yay!). I manually cut the side rails, which are 50x20mm extruded aluminum, and screwed them to the base using M5 bolts. The distance between the outer edges is 709mm now and they overlap the base by about 16mm on each side.

I also tested the design for the Y carriages, which are 3d printed in PLA and run on skateboard bearings. I think I have a working design now, but I'll know for sure when the gantry is rolling on the rails (hopefully tomorrow).