I have been playing around with an older Hohner 114-A, but this is a Cadillac compared to it. I am still trying to keep my thumb from slipping off and to keep the box stable when using the fingers on my right hand.
That seems to be the most difficult part for me so far. I guess I have to figure out my own way. I am experimenting with a strap going across my my body to see if that will help the stability problem. Having a strap off the right shoulder is hard just trying to keep the strap on my shoulder from slipping off.
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.