This prototype all steel holder is my own design, made from 15mm and 25mm tubing, which allows activation both by lifting the gauge spindle, or for greater movement by lifting the arm. The securing pin acts as a rotational fulcrum only, as when the arm is in the lowered position the arm is securely supported by both upper and lower end points in the top (angled) tube, so that any wear in the fulcrum has no effect on the reading of the gauge. In practice the non vertical angle of the spindle during the lowering action has a negligible effect when measuring plate thickness.
If a wider opening is required than shown in the "open" photo, this can easily be achieved by using a larger square or octagonal section for the top (angled) tube and increasing the length of the vertical section to suit.
Required tools are a welder, hacksaw, drill and a file
The simplest way to fit the fulcrum pin without the need for shims each side, is to securely wedge the arm in the down position in the centre of the top angled tube and then drill a hole exactly in the centre, smaller enough to allow for threading to suit the size chosen for the fulcrum pin. Then screw in a well fitting, greased fulcrum pin and hey presto, no shims needed and negligible side movement.
Note: This holder is more accurate with thick wall tubing than light, as it doesn't take much flex to affect a dial gauge reading.

Closed

Open

Back end

Front view