I recently purchased a NANUK 908 case for my VINTAGE concertina. An option with the 908 is a padded divider.
On the right and left, the padded dividers are fine and will snuggly fit an anglo concertina. The front and back, however, are too thick. I sliced open the front and back pads at the bottom and removed the styrofoam.
Replace the padded liner and insert concertina.
The concertina is padded snuggly on top, bottm, right, and left. The front and back are padded by two layers of material. Perhaps some very thin foam or other padding might be added. I plan to add spacers made from the leftover pads to block the concertina on both sides as it was in the original case.