Thanks for your input. I really don’t have the time right now to start truly from scratch, hence the use of the “guts” of another melodica to start with, just requiring the fabrication of new keys and a case. Daren’s printed instrument inspired me, and since the design for the keys has already been proven, a relatively simple re-design of the tops of the keys (relatively simple for someone that knows what they are doing, that is 8^P) could make it a relatively simple project. It appears that the key spacing on the Yamaha at the spring end is even, so designing keys should be straightforward.
I had not thought of the Schoenhut instruments. Might give one of those a look. They are certainly inexpensive enough, just not sure about the quality. I still want to prototype the keyboard before I get too deep. Probably a wood glue-up on an old instrument (I have an embarrassing number of instruments at the moment. I am very susceptible to MAS). But I think it would be cool to have an instrument for the blues that is as versatile as a harmonica-same pattern, different keys.
I probably will go with the Yamaha, if for no other reason than they are high quality and current. If the design works, and if others are interested, it would be far simpler to not have to redesign the keys for additional instruments. Not really thinking this will be much of a problem though 8^)