Question about repairing a toy Hero accordion

[Tim Hall]

I've got a small red Hero one-row seven-key melodeon - the "pseudo-Cajun" model where the keys operate flappers on the outside of the box. Like this one.



Whatever material was in place to seal the reed openings when the key is closed has now completely failed, and the thing leaks like a sieve. My question is, what should I use to cut new gasket material and improve the airtightness of the box?

Thanks in advance,

Tim