> random arangement of panels 2" to 4" in thickness , or should i cover the whole room ? <
For each corner take three panels and put them together to make one panel three inches thick, and put that straddling each corner. That's 12 panels. Then put the rest evenly around the room on the walls. If you space them an inch or two away from the walls they'll work even better.
That's the simple answer. If you want to get fancy you could place the eight panels to minimize first reflections off the side and rear walls.