Very interested in this idea. I knew there was some precedence for these kind of boxes, but there’s not many around. I’ve been digging around and i’ve found a few threads…
The Error instruments Spring Thing has a really nice feature, which is an audio input (example here). This would be pretty high up on my feature list for a DIY build, but i’ve got no idea how to go about it.
Very similar in design is the Electro Faustus Blackfly. The closest thing I could find to what I would want from this kind of box is this blackfly modified by UglyCasanova:
Sounds incredible.
Analog Industries has a tutorial for making this style of spring box with just a piezo, which looks like a great start, but I imagine a good part of what you’re paying for with these products is the amp section? And this would be where my primitive knowledge of electronics trips and dies.
Related, some time ago Electro Harmonix released a few percussion effect boxes that have since faded into obscurity. Had some great names, though - Rolling Thunder, Sonic Boomer, Super Space Drum… From what I can tell they worked on a similar basic principle - contact mic in an enclosure. The difference was they concentrated on building effects in. I’ve hunted down a schematic for the space drum, but you need to sign up to get access (which I probably will). Could be a good lead on extending the circuitry - though i’m far from competent, I love to brainstorm.