Yes, the sliding rod does seem like overkill. Its advantage would be that both the button and the vent hole are on the end pieces. Plastic piping is a good idea, maybe combined with a lengthened push button. My woodworker (Rick) suggested teak as a kind of wood that would withstand moisture, but lacquering up sounds right. My first thought was sandalwood. I have a sandalwood Buddha I bought in Myanmar, and it has a wonderful natural aroma (not having been artificially odored up as is often true of sandalwood beads).
I like the kind of combination of wood and other materials in your Banarsë, especially the mouthpiece (it will be interesting to know how that holds up). So we’ll consider different possibilities, possibly for end plates or the top plate, which could be metal or plastic. Rick pointed out that the thickness of the wood will be relative to the extent to which it is supported by the rest of the structure.
The selection for the inner pre-existing melodica core will, it seems to me, constrain or encourage decisions about the rest of the design. So I think our first step will be to familiarize Rick with a variety of melodica models and designs, including some vintage instruments.