Waterproofing Multiflex Tailgate

[KevO16]

So I installed a Retrax roll-up aluminum cover, which does a decent job of keeping water out from the top. The issue I’ve noticed is water coming in between the multi-flex tailgate upper section and lower section, which gets my bed fairly wet, defeating the purpose of the cover. I can see straight through the gap between the two sections. Anybody tried to waterproof their tailgate more? If so, how’d you do it?