The solution!

Could not find any glue which purports to work on plastic sheeting with any reliability. Same with tape, at least for outdoor use. Not much of a greenhouse with holes running laterally across the roof where the plastic overlaps.

Came up with the solution this evening. Duct tape will work! But not just taping the pieces together. I have enough overlap that it would work to tape one or both edges, then roll them together and fix them to the sides. This would probably work without the tape but I think the tape will help the rolls lay flatter and add strength to the fixation points, which would be more prone to weather stress. 

It will require some disassembly and reassembly but not taking the whole top down. No more hardware needed and I have the tape on hand. Can get this done in two hours or less.

It does rather amaze me that there are no glues or tapes made for this use.