We have landscaping rocks in my backyard. My Intex pool will only be up in the summers, so I have no intention of moving or eliminating my rocks. The area is level , but I want to make sure the pool liner is safe. Any suggestions on what to put underneath? I was going to set down ply wood or sub floor squares , then a tarp and then the pool matting or a foam pad....the pool is the 24x12 Intex ultra xtr. If that helps.