Working on thin parts in the milling machine creates a whole new set of challenges in workholding. Thin metal and plastics do not appreciate being chucked in a vise...they bow and bend in ways that do not promote the desired end result of flatness after machining. For some jobs, the only practical solution is a vacuum chuck.
This particular chuck was made mostly of scraps, which is why it is only 6" square, but in theory there is no limit to the size. There is nothing magic about the materials - just what I had sitting around. The top is a piece of aluminum jig plate from a yard sale (ground to a flatness tolerance during manufacture), and the vacuum chamber was milled from a piece of 1.5" thick plexiglas.
I left an unmilled boss sticking up in the center of the plexiglas to help prevent atmospheric pressure from depressing the aluminum plate out of a flat condition. In use, you place the part on the chuck and then mask off the uncovered ports with tape so that you don't reduce the vacuum available to hold the workpiece down, A small diaphragm vacuum pump is all that is needed to create the hold-down force.