Isn't there some kind of water proofing spray you can put on the circuits? I guess you'd still need some way to protect the batteries, but I think it is doable.
"conformal coating" is the thing to google, though that only protects the PCB and not the tricky bits. You'd still need to figure out a way to waterproof connectors and buttons and moving bits like that.
But neither injection molding nor carpentry will protect a synth from a child dunking it in a puddle of water.