Rain getting in bed cover

Have a 24 1794 Crewmax. Neighbor has a 22 platinum and changed to a camper shell. He was nice enough to give me his bed cover that was factory installed from the dealership. Fits great but rain is getting into the bed. My best guess is that rain is getting in where the tailgate meets the bed and is running down the rail and down into the bed. Anyone have a recommendation on what to install to keep the water out?

 

I had that style of cover but as a bakflip. I had it for 8-9 years on my Gen2 and due to the flushmount design honestly never found it to be as water tight as a top mount cover that sends water out over the rails before it gets to work at the seams and cracks. But I was able to get it down to a small enough amount to satisfy me if it was perfectly centered and adjusted. Did yours come with the drain system? Should be a nipple somewhere at the front of each rail that sends water out of the bed via drain tubes that would go out through the bed drain holes.

Tailgate seal kit at those corners would probably also help.

 

So I had a backflip prior to this one, no issues at all with water getting in and had the drainage system. Best I can tell, this one is the toyota factory one. It looks to have a drain line system but it looks a lot different than the one I had one my ford.

 

Adjust to the rear if there is wiggle room to so it and/or add tailgate seal?