I'm sorry, i dont get it, maybe its becouse im dutch and there is some misscomunication, but you where speaking here about 2 panels on the left side and 2 panels on the right side, correct ?Something pretty thick on the rear - at least 6" and at least 2 2x4' panels.
For your side walls you'll want more surface area and thicker. Again 2 2x4's each side and at least 4" thick with a bit of a gap behind them.
Do i need the second panel on the same side to mount right next to it, above it, or would there be another way to check where to place? for as far i could check there is not a second reflection point on the same wall to find with the mirror technique.