On my Atlas, they started off as ptfe gib strips (which I now think is standard issue), but they can be almost of anything, brass, bronze (recommended) or mild steel, and yes, they will need to be a nice constant thickness along the length. Nothing too exotic, but you do need to angle the narrow edges so that they sit in their position nice and snug, with only about 0.25mm (0.010") adjustment gap.
Also put countersinks into the wide side for the grub screws to sit into and hold the strip in position even if the gib is slack.

John