Here’s an extended backing with spots for two antennae on the top. The NRF24 module blocks one of the antenna ports on the back of the plus and makes the other one a tight squeeze, plus I don’t like that you can’t use the magnetic mounts when an antenna is installed on the original back.
It still has the four posts for magnets to connect the back to the device but I’ve raised them. I’ve also added six spots for additional magnets for mounting, I wasn’t sure if four would be enough for the added weight. The magnet slots are 8mm in diameter. I used the 3D files from the T-Embed Git so everything fits nicely and matches the original.