@Eldon_Joh
I don't understand the flat plate on the bottom.
What you want is another closed box about the same size and shape as the Z column.
I went through several iterations looking for something simple and (I hope) sufficient. One of the first was close to your design, though retaining the large plate for the added mass. Then a constructed box and then just the welded on extension. Moved to the bolted 2 plate configuration thinking that the benefits of adjustability outweighed the added complexity. In any case, FEM seems to indicate that the simple welded on extension won't measurably deflect -- well under a mil with an applied 150lb static load.
Is there any reason that stiffeners or a box support could not be added later?
Is there a build thread for your Frankentag? What level of precision are you getting (deflections at the spindle nose when milling different materials)? What size linear rails are on the vertical axis? Did you look at the SC-15P epoxy by Precision Epoxy Products?
@Ian
Thanks for the comments. If there was one change to be made, what would make the most difference in terms of ultimate precision?
Thanks!