I also had difficulty in stabilizing the box. I finally tried a two strap system like the larger accordions and it worked better than I hoped. I didn't even have to use the thumb strap and thus was able to move my hand up and down the keyboard. This kept my wrists straight and eliminated the pain in my wrist. It was a very comfortable setup. You will need larger brackets but it is worth it. Just a thought. Good luck and keep squeezin'.
I used to have a problem with the strap slipping off at one time.
Then I got a strap made by Danny Dyson with the "rough side" of the leather against my shoulder.