DIY Riser Block for 6" x 26" Import Mills (Grizzly, Enco, Harbor Freight, etc.)

For all the theory, all of it good, the only way to be sure is to try it. Or, since I lack large enough equipment to machine one from a solid block, I could pay a local machine shop to do it. Where's the fun in that?
The problem with that design is you're sacrificing quite a bit of rigidity by using a 3" diameter pipe rather than a 6". Best design would be to use a riser of 6" (or more) in diameter with the bolts going from the turret to the column through the riser. That way you put the riser in compression and the bolts into tension, limiting how much each can flex.

In the same material and wall thickness:: the 6" riser is 8× stiffer than the 3" riser.
In the same material and solid:: the 6" riser is 16× stiffer than the 3" riser.