The Mellotron Group

Though it may seem ridiculous your idea makes some sense. I've gotten some sympathetic tone ringing out of large marimba notes that I've built. I tried miking them up and it added a warmth to the sound if not an actual reverberation. I wouldn't use a sheet of ply though- the way it is laminated prevents it from resonating. If you glued some 1/2 thick cedar lengths together on edge ( and better if they are quarter sawn), hung the "plate" from its nodal points and plcaed a speaker and a mike close by you might get an interesting tone, if not an actual reverb. I've seen Mr. Beith's lovely plate units-this is something of a wholly other order!