I am not clear about the leak, can you clarify? Is it leaking even when the top is down, or only when it's up?
If it leaks
when it's down, and you never care to have it up anyway, perhaps it could be sealed 'semi-permanently' with something like Eternabond tape. The Eternabond will not really come off (thus the name) and its sealing ability is excellent, but I think it could be cut at the joint with a blade by some future owner who wanted to raise the top again. The tough part would be how to properly tape the corners, though.