For my home build I'm planning on 8" of travel. I was planing on using a 1/2" aluminum plate for the moving part of the Z Axis, mounted to 4 HWIN 15mm bearing blocks. So the router bit to the first being block pair is about 8.5"

How narrow, width of plate without a gusset, can I get away with assuming nothing more than a 1/2 "deep cut in plywood or MDF?