Is it an English concertina or an Anglo? The straps being different, with English having thumb strap and little finger rest and Anglos usually have strap across hand. My English (which does require strong thumb joints!)did originally have a very fine neck strap attached to two rings on the top of the instrument. I guess that could be rigged up for any concertina, but would not allow for much movement - you would still need to support with your thumbs. It's possible though when sitting down to support one side of concertina on you knee. That might be your best bet.