The Scooter gauges mounting bracket accepts any 1/4" bolt or screw; like the one on or near your switch housings.  As well, some bikes have bolts holding the mirror brackets on; this is a perfect place to "float" your gauges.

ELA-20710 Thermometer (Matching Clock also Available)

On Harley Davidson motorcycles, the switch housing screws are recessed and we have considered that in the mounting hardware.  By notching the end of the chrome mounting bar, we have made a key like fit that's made just for that recess - and it locks the gauge firmly in place.

 

Black Police SL Shown