Back in the day, before the dead sea reported sick, I bought some sucker windscreen temp gauges from Halfords.
They looked silly on the windscreen so I left one in storage and used the other 2 indoors as they proved to be very accurate.
I've found the bottom of the front cup holders to have a flat base and is perfect :
Of course that vent is closed off and with the sun in the wrong position it does misread, but, in the main it's useful.
Sits and sticks perfectly :
Indoors, as I've had for years :