Designed this mount in Fusion 360.

It is modeled to fit the Official 7" display and mount it to a 3 gang wall box.

The frame attaches to the display using M3 screws, and the screw to lock it in place is an M2. There's a slot on the bottom mount for an M2 nut, but the holes are tight enough you could probably do without it. I'm using a POE to ethernet / microUSB splitter to power mine and connect it to the network. Right angle adapters are definitely required, unless you don't attach the Pi to the back of the display.

I'm including the STEP file for anyone who wants or needs to modify it.

Edit:

I've added a V2 of the frame to allow for a USB port mount. I realized that if I was going to put multiple of these in my house I would want a way to hook a keyboard and mouse up to them without having to take them down.

I used these to solder my own USB extensions, and they fit and work perfectly:

https://www.amazon.com/gp/product/B071JK5614 <- Female end, that fits in the frame

https://www.amazon.com/gp/product/B072MCHX41 <- Male end, plugs into Pi

I also slightly adjusted some of the mounting holes to make it all line up a bit better, but either model works just fine.