I have had some success with tent seam sealant, which I just happened to have next to the box - and made the connection. It has make a big difference, although it was something of a fart to do.

Had to fix it with the bellows extended for 24hrs for the glue to dry.

Next plan is to see if I can blank off one set of reeds because I don't like the tuning, see if it sounds better - and the air goes further - with one set.