After spending a few days in CAD, I came up with a simple solution. Then I placed an order for materials, and choose a 6"x6" aluminum angle with 3/4" wall thickness. This is much more economical than hogging out a large rectangular block of 6061!
The front & rear plates will be made from ALCAST-5, which is the Canadian equivalent of MIC-6 jig plate! The Canadian plate is much nicer material to work with, as it is not a chalky or powdery as MIC-6! It works much like 6061!