Honestly, I've never seen a Tonneau cover that's totally leak-proof under all conditions. The only way to make a tonneau cover totally leakproof would be to seal the folding seams and edges with silicone sealant (which would make it unusable
). Just accept that most modern tonnneau covers will keep out 99 percent of water under most conditions. My new Extang Solid-fold is pretty water-secure and I've only seen a few drops enter when hitting the seams with the high-pressure hose at the car wash. I think it's fair that's all I can ask of it when it's designed to fit so many vehicles, is easy to open, and easy to take off.