That thing is a big PITA, isn't it?
The mopar lube works fairly well for me, but still far from a bulletproof solution. I sometimes will attach a small carabiner clip to the zipper to get some leverage on it and not scrape up my hands/fingers in between zipper and top.
I think the most important thing is getting it lined up just right before zippering it up; sometimes mine is a major pain, and other times it is as easy as the sides to open (which I have absolutely no problems on the sides)... All depends if I am off a tooth or three in one direction, it seems.
Easy solution - don't put it in from spring - fall. LOL. Just pull drain plug in your rear cargo areas and the storage bin.