To survey using an optical level and survey staff needs a perfectly horizontal level and a vertical staff.  This helps you or your helper keep the staff perfectly vertical when taking readings.

Designed in Fusion 360 in 30 minutes so don't expect miracles.  Designed to clip on to my kit which is 53mm wide, 29mm deep and 19mm side before it turns in.

The bubble level was from Amazon and (bizarrely it's a precise imperial measure at 1 ¼ inches across).  The bubble is a very tight fit, deliberately so.  It's not coming out!

I will upload the fusion file once I've sorted out the parameterisation.