Using 1-2-3 blocks, I could raise the part off of the table and lock a bar underneath the part to the table, then use the bars to anchor the part to the 1-2-3 blocks and the table. I chose to order some 1" x 0.75" rectangular bar stock (36" long). The height could have been 7/8", but absolutely had to be below the 1-2-3 blocks small dimension because I didn't want to mill these bar clamps. This long bar was cut into 8" long pieces.
I marked the center line of the bar on the wide surface (this will be the top, so the bars are 1" wide). Once that was done, I marked the center point. My table has three t-slots in it, so I used the center mark as one hole for the t-slot, and marked the other two t-slots in distance. Once complete, I marked a second set of holes on the outside of the table (and subsequently the part I needed to anchor).
These were drilled out with first a #18 drill as a pilot hole, followed by a "Q" drill bit (the one for tapping a 3/8-16 thread). The middle three holds for the t-slots were drilled to a "W" for a close-fit through hole - this would allow these three holes to anchor the bar to the t-slots.
I then took the parts to the mini mill with the "W" drill bit. I'd use the drill bit to center the hole for the spindle, lock everything into place (e.g. clamp it down, and all axis' except the vertical locked), and then swap out the drill chuck for a 5/8" collet and end mill (perfect size for the socket cap 3/8-16 bolts I would use). The hole was then countersunk to get the head of the socket cap bolt to fall below the dimension of the 1-2-3 blocks (clearance, see the previous photo).
With that complete, I could then bolt everything down to the mill table and proceed to mill a flat surface.
I would call that a success!
