Low profile clamps.
Low profile clamps are very useful for holding flat slabs of metal to the mill table, see photo above. They allow the whole surface of the slab to be accessed by the cutter. These low profile clamps are very easy to make.
This shows an alternative style of clamp. This one is only 6 mm thick and the clamping screws are small M4 grub screws that act on push rods,
Also not the the clamp is wider and the central securing hole has been elongated to a slot. The is enables the clamp to be used parallel to the tee slots on the mill table. The slot provides some opportunity for position adjustment.