Thanks guys.
If you make one, test it on scrap first. As I mentioned above, the part did move when I milled it due to clearance in the notches of the fences. The part definitely needs to be captive at the corners. When I machined it I had 25" of Hg. You have to watch the gage to make sure that the vaccum holds while the part is machined. I started with .005 cuts and did manage a .010 cut.
Some design notes: The ports are counter sunk so that they can be plugged with wax* if need be in the future. When the need arises, I will drill two more .250 dia holes parallel to the existing one and drill additional ports. The .250 holes will be linked by a manifold.
*Electrical tape also works well for sealing any unused ports.
-Bob