Cut the 12x12" plywood sides, and put the 4x4" section 2" in from one edge, and 3" in from another edge. Cover the 4x4 with cheap cut-off carpet from the big-box stores. That gives you shooting heights of 10", 9", 7", and 6", not counting the thickness of the carpet.
The 4x4" widths are 15" with one that's 10", for small people and small rifles.