A simple and easy to make adjustable gauge can be made from 12mm x 1.5mm square aluminium tubing, (or a similar size, sufficient to give both lightness and rigidity), a 3/16" gutter bolt, or similar, can be used to adjust for the required height at the bridge point (usually 28mm with the fingerboard fitted, for violins).
If a smaller contact point is desired the gutter bolt can be reversed, (or the head cut off) and the shaft rounded off and polished, to prevent scratches.
The tubing is cut to a length of Neck Stop + Body Stop + approx. 1", the gutter bolt can be fitted with a 3/16" hole and two lock nuts, or a more elegant solution is to drill a smaller diameter hole and tap in a 3/16" thread, it then only requires a single lock nut or thumbscrew.
With the centre of the adjustable height bolt positioned exactly at the bridge point the excess length is cut off where the fingerboard meets the nut, to ensure the tubing rests only on the fingerboard to ensure accurate neck angle adjustment, as many fingerboards are sold with the nut already glued in place.
The same formula can be used when making gauges for larger instruments.