Unfortunately magnets are not going to be strong enough.
It has the same problem with the tensioner unbolted... I did not think of it before, but the lid had been flexed because it appeared that at one point the tensioner was removed and when the old owners opened the lid they had let it go and it bent the hinge and lid where it connected. We tried flexing it back into place as good as we could and then bondo'd the heck out of the top to make it look level. LOL, should have been my first clue that I was going to run into problems getting it to seal right on that side.