I have made a lot of modifications to my car, but I have tried very hard not to drill holes everywhere.
So I used some Neodymium 32 x 8mm north south pair of magnets with 25kg pull force. They are less than 25kg as the magnets are about 3 to 4mm apart through the plastic trim panel. If the plastic is thicker, say 6mm you will need something a bit stronger for a 7 inch GPS.
I did buy a few other sizes but this one worked out about the right strength.
The 32 x 3mm strip of aluminium about 150mm long is to stop the GPS vibrating on the slightly flexible trim panel when sitting at the traffic lights with engine idling, there was a specific spot on the trim panel which had a poor nodal point! M4 counter sunk bolt tapped into the aluminium, plus some Loctite.
I had to Dremel (Bosch 10.8v equivalent) out the back of the GPS mounting bracket to recess the magnet.
I tapped an M6 hole up the middle of the bracket & screwed in an M6 nylon bolt, then screwed a 4g screw into the head of the nylon bolt, pre drilling a hole for the 4g screw, to hold the magnet on.
(The mess on the back of the trim panel is acoustic insulation I have installed, another topic in itself)
