I wanted to mount my Amazon Fire HD 8 including its cover (https://www.amazon.de/dp/B01N9880VN/ref=cm_sw_em_r_mt_dp_U_LEMFDbCE60NAS) to the wall as a permanent wall screen for home automation.
I came up with this simple design that works fine for me and could be the base for your own thoughts.
There is a hole prepared for USB charging - how this works:

• after printing put the tablet in place and put the usb cable in the tablet
• now move the tablet aout 0.5-1mm out of the holder again and make sure the USB cable is still tightly connected
• fill the back where the USB cable comes out with hot glue
• let it set and you are done
  Now, every thime you put the tablet in it should slide right into the USB-connector.

I also included the FreeCAD file - so you can start over from where I stopped.