We could start with one – an adapter which enables Suzuki mouthpieces to fit Yamaha melodicas/melodions, as it sounds like this is your greatest need at the moment?
Yamaha mouthpiece connectors are 12 mm long, and tapered – they start at 13mm diameter, and taper to 12mm at the end.
The Suzuki mouthpiece connectors are 15.5mm long, and have a 13.5mm diameter, with a rubber ring, around midway, extending the diameter to 14mm.
I don’t have a Suzuki melodion at hand to measure the mouthpiece aperture, but I suspect it’s around 14mm. Could you measure and confirm?
So we need an adapter with a 15.5mm long aperture which has a 14mm diameter (to be confirmed, as soon as someone measures), and a plug of 12mm length, tapering from 13mm to 12mm.
I could design one of these for 3D printing, and have it available to order directly from the manufacturers. They have this facility already set up, and its easy to use. You would just have to pay them directly for the manufacturing, and they send it to you by post.