I can't speak to straightening up panels, but I've had to do that with several pieces of Boy Scout fabric I've been working with recently. I used my old folding cardboard cutting board that I used for sewing patterns and pinned one selvage edge in place, then pinned the opposite side, making sure the pattern lines were straight. Then pinned the other two sides. In one case I started with dry fabric and sprayed it with water after pinning, the others were wet to begin with. Then I just let it dry in place. Time consuming, but it worked beautifully.