The supports for an accordeon tend to break after some years.
There are (unfortunately) several types. This one is for larger (conservatory) instruments.
Note that the hole for the screw is not in the center. There are two possible pieces which are a mirror image of each other.
These pieces are available in accordion parts stores but seldom in stock and stores are few and far between... so unless you live near Castelfidardo in Italy making your own piece is easier than buying it.
This was created in Google Sketchup.
You may need to drill out the hole for the screw a bit but other than that it's self-explanatory