how do we keep it from bowing in the middle when gripping.
Brian

Dont "grip it".
Clamp it down to the table gently with many clamps.

OR, hold it down with vacuum?

Sometimes double faced tape works for me, just watch how much load you put to it.

Yep, double sided tape works for me too but it scares the shoot out of me trying to guess when the coolant (I use WD40) will weaken the bond enough for the end mill to grip the part and lauch it in my general direction at 3000 RPM lol :)

What's the size of the blocks? What material are they made from. And is there any holes in them?

when i was in aerospace while milling thin aluminum we used endmills with negative flutes, this will push the mat'l down while milling instead of it trying to pull up the spirals of the flutes