How to seal gap between bed wall and tonneau cover

So I got a 5 foot bed with a TruXedo TruXport tonneau cover so that I could essentially use my bed like a trunk and store cargo in it that I don't want to get wet. It works alright, but along the seams, especially where the tailgate meets the end of the cover, there isn't a tight seal and during heavy rain water gets in. What's a good, cheap, easy way to ensure no water can get through? I was planning on just sticking a cheap beach towel between the cover and tailgate in the mean time to clog the gap, but that would really be a temporary, stopgap solution. I also don't want to replace the cover, it's nice in every other way. I can't see how any other cover in my price range would be any better. Does anyone have any suggestions?

 

got a picture? Not sure how that cover meets the tailgate. It doesn’t have a rubber flap of any kind?

 

yeah. Sorry, it's dark where I am right now so the photo is crummy but you can kinda see how it's set up (on the left side is the cover, on the right side is the top of the tailgate, going up the center is the seam). There's a rubber flap but it just tucks into the bed and lets the water come in, and it's not like it's some high-tech mechanism that has the potential to be super effective should I position it right. It's a very thin flap and very short, and on its own won't be able to seal water-tight.

 

I cant tell wtf im looking at but i had the same issue at the front of my box, picked up some foam tape and boom fixed