sorry for the late reply.
RE: I want to do your design but how hard would it be to put a back like this. Except replace the amazon logo with the raspberry pi from the original design and put in the two should buttons.
Not sure if you can extrude from the edges as the source file has rounded corners…you can give it a go and see if it shows up as a weird render. Have a look at youtube, before I made the mods I’d never used Sketchup, but after watching a video and doing some test changes, didn’t take long to pick it up.
Think it was this one: https://www.youtube.com/watch?v=Hb-Adao_Q1c
RE: Also how hard would it be to put USB ports into the side?
I managed to add in an audio jack so if you measure up the size of the USB port itself you can double check that it can fit…then just cut out the shape from the existing model, again the youtube video covers that kind of stuff.
However that being said, you may run into power issues if you try and pull too much juice from the Powerboost and have shorter usage from the battery, or also run into issues of not being able to power up the USB device you try and connect….I ran into an issue with a Bluetooth Dongle not powering, so fell back to an audio port instead.