I have started building riser blocks for the gantry. After looking around at my materials on hand, I decided to use some very heavy duty 2x2 square steel tubing. Not as easy to deal with as aluminum, but it will be plenty strong.

I took off the gantry and noticed that there is a recess machined into the base plate of the machine where the gantry columns go. Presumably, this eases assembly and ensures alignment during manufacturing. The recess is about .063". On the risers I have machined a mating protuberance.

With the risers, I'll have about 3 1/2" clearance from the table to the Z bracket. I can always make taller risers at some point if need be.

The work envelope is ~24" X,  12" Y and whatever I work out for Z. I'm thinking of going for about 4" travel in my mechanism.