I had to do this a couple of times and I used "Crazy Glue". I know, I know....I thought the same thing...but apparently the chemical that Crazy Glue is made from is cyanocrilate(?) or something and it reacts with the plastic wrap and supposedly kind of "welds" the plastic back together. I used the gel and it comes in a tube with a small needle-nose -so you can squirt it down into the gap a bit. You might need several tubes of it...I think I used about three of them (they're small).
I applied some blue painter's tape to the exposed areas of wrap along the seam so I wouldn't get glue on that part when I clamped everything up.
To clamp, I used a couple of pieces of flexible plywood pieces to protect the inner shell from the clamp ends and then on top and along the full length of the seam, I ran a 3/4" X 3/4" maple strip of wood in conjunction with the clamps so that the pressure would be evenly-applied. Since pressure will still be greater at the ends (nearest the clamps), I "thickened" the central portion of the maple strip with a few pieces of painter's tape -so that when the clamps were applied, the pressure would be more equalized along the length of the entire seam. I hope that makes sense. The Band
Leave the clamps on at LEAST overnight so that the warped plastic has time to re-shape itself and straighten out. Then, remove the clamps and check to see if there might be some gaps left...repeat as needed.
So far, the Crazy Glue has held and all the seams are nice and flat now.
Good luck with your project! :)