Welcome, Gh0stC0de. Have you seen this one?
There are a few other cigar box melodicas out there. I searched for a book on making wooden musical instruments and found Making Wood Folk Instruments by Dennis Waring and Handmade Music Factory: The Ultimate Guide to Making Foot-Stompin’ Good Instruments by Mike Orr. I have the latter. Both look pretty good but don’t seem to contain much that would apply specifically to the melodica.
Among cheap melodicas, I like the Study 32. But the idea of building your box around one and swapping it out later sounds inadvisable. It would probably be better to start out with a Suzuki M-32C (or a Yamaha P-32D, which is what Daren used for the Banarsë — to some people’s ears, the Yamaha has somewhat more of an acoustic sound).