This looks like a pretty good design, but you might want to rethink some details. Mounting the Y-axis rails like that will be problematic; you're depending on T-nuts to hold the rails absolutely straight, flat and parallel to each other, which is a lot to expect. I'd suggest making up a plate that would mount to the extrusion, with channels milled into it, which would provide a flat base for the rails and ledgers they can rest against to ensure straightness.

Also, those red-colored mounts between the base and the extrusion will have to be machined to exactly the same height and absolutely square, or the whole thing won't cut straight. Do you have access to a milling machine? I'm not seeing this working too well without one.