I have an Avatar 2X12 Traditional cabinet with a closed back. I would like to convert it to semi-open back. My choices are: 1. Buy a new back from Avatar (partial open oval back) - for about $55 shipped. 2. Cut an oval shape out of my existing closed back rear panel. 3. Buy a couple of 3/4" 2X4's that would cover all but the middle section of the rear of the cabinet. If I go this way do I need to use baltic birch plywood or maybe just use any hardwood (pine or hemlock)? How important would getting void free just for the back portion? Thanks!