Thanks for the encouragement, guys! I’m not thinking of anything comparable to the innovation of Troy’s Banarsë. My idea is to get an existing good quality melodica functioning well inside an attractive, artisan produced wooden case, see how it sounds, and then perhaps make some alterations on the basis of that.
One of the challenges, as illustrated by the Mylodica, is the spit valve. The first Tokai Gakki and Yamaha Pianicas had a sliding rod, with the push end by the mouthpiece, that might work. These Pianicas are from 1961-62 (+?), have solid internal construction, and produce good sound. Here’s what the spit mechanism looks like, and in my experience it’s an excellent mechanism and vents extremely well.