I think I have done it
So I decided to go with used Nexus 7 2013, due to fact that it has build in wireless charger. In the beggining I was thinking to use OEM charger, which is magnetic and it should be able to keep tablet on it (attached video above), but I can not find such charger nearby, so I decided to go with cheap QI charger + magnets.
What I bought:
This charger is quite big, so it is fine for us, because there are plenty place to put our magnets in it . I have put circle one 30mm diamiter, 5mm thin with hole inside to screw it + 40mm x10mm and 5mm thin.
It looks like follow:
It is important to get magnets with hole to screw it, since this material is very sensitive to drill it. This holes also let me screw and keep charger on the wall. I have not used miniUSB port but use power cable which I had on this wall and put 2A charger in tech room nearby.
This charger is quite thin, ca 7mm, but I have remove few mm from wall surface, so it looks like build-in
Then I have used silicon cover from Nexus 7 (std one, also very cheap),
I have used electric mini polishing machine to prepare place for metal pads.
Those pads I have taken from magnetic holder which I bought. There were two pieces of pads, one ca 2cm x4cm and circle one ca 3cm. There ere 0.5mm thin, but then I polish internal surface of cover I had good place to put it inside. I have done it because such metal pad can make some scratches on tablet surface and I have feel thicker places when I take tablet with this cover. Then I used double adhesive tape to stick metal pads. Those pads was painted, so they are not such ugly when you will look inside cover
How it looks on the wall:
The biggest advantage is convenience, you just need to put it on the wall, putting more or less in the middle. That’s all. Also when I need tablet I can just remove it quickly :).
After fighting with iPad2 this Nexus 7 is much better device, it is smaller and much faster. Charging works very well, just after I put table on the wall, anyway I have must check my USB charger (Idont believe this 2A on it), since my rapsberry didn’t want to work with it. When I switch on motion detection, charger is not enought to charge it, probably due to battery drain from motion detector.
Second problem is that tablet is quite sensitive for magnets, it is better to test where to put magnets before, because there are some places that table switch off automatically just after switch on. This place which I used is free of this problem.