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Many years ago (1998) I had a topper installed - they used a double layer of thick/ double sided foam tape, approx. 3 inches wide. After 12 years (or so) the tape started to work its' way out in places near the cab.
Even so, it still offered near water penetration resistance.
Go to any topper dealer and buy the foam and stick it to the bottom of the topper, on the front you might have to double it up to seal up the gap caused by the plastic bed covers..