The simplest way of achieving what you are looking for is to buy an aluminium tax disc holder (the ones sold for motorcycles). Bend it through 90 degrees over a broom handle and fit a lift-the-dot fastener in the small hole. A leather pad on the back will protect the car upholstery. You can buy self adhesive sat-nav mounting discs quite cheaply if the surface you end up with doesn’t suit your device. I cannot upload a picture as I have given my version of this away and moved on to a more refined mounting using a fixed mounting for the sat-nav. It still fits on the lift-the-dot peg for the tonneau cover.
