I found this to be useful for keeping the buttons in place for my Smartbox AAC. Simple velcrow strips can be used on the top to keep the buttons from moving.  The case is held closed by using 5mmx3mm neodymium magnets from Amazon. This allows the bottom to be easily removed so you can push the power button as needed. Be mindful of polarity when putting in the magnets, it can be a bit tricky due to the small size and dealing with glue. I made a bit of a mess as you can see from my photo. You can also add little rubber feet to the bottom to keep it from moving around on smooth surfaces. 

 I used PETG but any material is fine for this print. 

Fusion360 file is included so adjustments can be made as needed.