"Resonance" is the key word here. I never used one, but I assume standing or sitting would make a difference. I'd probably start with something with a 1" lift/gap at one end and 2" plus at the other--with solid sides, cut at a taper. And maybe start with a 1/2" plywood top and see how that works--you can always go heavier or lighter, depending. I'd put a bottom on it so it doesn't collapse, reinforcing the corners. As for size, it's your call.
I think Catfish Keith uses one, so you might check out his videos.
