The Y-Axis rail is the track upon which the Y-Axis assembly will slide. Cut two aluminum angles the length of the Y-Axis rail.
As with the front of the Y-Axis assembly, there is the same "flat spot" on the 45 degree beveled top-and-bottom of the rail.
The aluminum angles just sit on the rail. They will get firmly captured in place and will not move around once it's all put together.
Here's the Y-Axis assembly and the Y-Axis rail together, from two different views.
Here's my initial test-fit of the Y and Z-Axis components. In this early version, I had used wood screws instead of the cross-dowels. After lots of screwing and un-screwing, I converted to the cross dowels. So much nicer.
You may be wondering why I used plywood instead of MDF (Medium Density Fiberboard) like everyone else. Being basically cheap, my next-door neighbor had just finished a kitchen remodel and he gave me loads of left-over plywood. Pretty easy decision.