The Grand Slam has a round center hatch, right? (never seen one before..)
If so, check to make sure the seal is in good condition. It should rise up to make contact with both parts of the hatch - the ring and the lid. I'm also unsure of this, but it might need the bag there to help make the seal. You can experiment easily with a garden hose...
The front hatch I've never been impressed with because its held with bungies. Even nylon strapping can be problematic. I'd probably make one out of fiberglass and be done with the original.