Another method that sometimes helps with "regular" width cotton that is off grain is to stretch diagonally. The problem, of course, with such a large piece is that you will need someone to hold the opposite corner. You may need a couple of goes. If you do an internet search for "stretch fabric diagonally to get back to straight grain", you should be able find videos of it being done. Panels, imo, are generally horribly skewed and this usually helps.