If you want to do it with a Fairphone 3(+), there is no risk to do so. The silikon rectangle inside the cover holding the plate in place.
I do it the same way with no issues, as there is no open contact like in the FP2.
I would be careful in any way. Every millimeter in a smartphone is thought of and that plate must be as thin as possible. Sticking it on the outside of the backplate or the inside of a cover feels safer when concerning the space, but - as you mentioned - it will do something to the backplate/cover. It’s your choice.
That’s an interesting idea.
I use Velcro strips stuck on the outside, but not protruding beyond the protective rubber ring.
I have them in various places around the house as well as the car where I like to stick my phone - e.g. the front door where I can run a security-camera app like IPWebcam to record if anyone came while I was out.
On second look, I can see metal strips running on the back of the phone (under the back of the plastic cover). So even if my metal plate (quite thin) is painted, I’ll rethink it.
My problem is that the plier (?) in my car connected to a suction cup doesn’t sticks really well on the windshield , contrary to the magnetic holder I can put in the ventilation output…
I’ll resort to some light double face tape, then.
Thanks for all your answers!
I have a magnetic support in my car and a wallet cover for my FP3.
When I get on the car, I slide the metal plate in the cover internal pocket (which is not permanently attached to my phone).
It sticks good.