Early on, I got a Stagi G/D concertina, which is a bit big and heavy. I found that it was hard to play standing.
So I looked for some way of supporting it against my pals palms. After experimentation, I found that two boxes for blues harmonicas were about the right size. So I got a piece of wood of the right dimensions and cut off two pieces 3" x 1.5" x 0.75". I glued on chamois leather covering them completely (rough side out), like wrapping a gift.
As they are loose, I use them on all my concertinas. The chamois has just enough purchase to stop them shifting, and won't shed dust into the mechanism. The weight of the instrument is enough to hold them in place.
Of course, this assumes a wrist strap. No use on an English concertina.