SOUNDS TO ME LIKE THE COVER MIGHT HAVE BEEN HOT LIKE IN OR NEAR A FIRE AND SHRUNK, COMMON IN BOATS. I WOULD TAKE SOME LONG WOOD AND WEDGES AND VERY GENTLY APPLY A SPREADER MOTION COMPLETELY ALONG THE INSIDE OF THE COVER, ONT GO FAST AS IT WILL CRACK ALSO CHECK TO SEE IF ITS WARPED........
