Help needed with clamping workpieces

Hi, I am fairly new at this hobby and after getting all the toys setup and some g-code put together I started to make chips.

One issue I am having is with clamping the work piece, for example I want to mill a 3" flywheel from a 3.5" square by .25" of Aluminum. I can clamp the Aluminum to the table on top of a piece of sacrificial wood and start milling but when it comes down to final pass the cutout will start to move as the bottom layer of metal is removed, this will at least put a an unwanted "dent" in the flywheel or at worse bury the mill into the flywheel.

Any suggestions?

ex-egll