This is a 6" diameter Spherometer I am using to grind a 9.25" mirror.

Each leg from point to point of the ball set screws is 131.98mm.

To make one you will need:

(1) Printed Spherometer body. (3) M6 ball tipped set screws, 16mm long.

https://www.mcmaster.com/93339A211/

(1) Economy .001" resolution dial indicator.

https://www.mcmaster.com/20715A74/

(1) Any M4 screw to secure dial indicator in position.