Wood disk routed to press fit in the bottom of a coffee can, old micrometer with the arm cut off.

I do not use it to measure the sagitta, just take readings across the mirror to see how spherical and astigmatic it is.

The can bottom seems pretty round, but just to be sure I put a mark on the disk and always orient that toward the edge of the mirror.

You could also do like some of the foucault testers, use a 1/4x20 threaded rod and a dial marked in 50th's to take readings to 1/1000".