Demeter,
Can't help with the film, it's essentially chemically inert so solvents are out. The film should come off even if heated on. If it won't - it got way too hot, it shouldn't melt on at 300F. The reality is IME that your going to have to pick off little pcs at a time or if it's a small pcs and not structural or glued - stick it in the freezer for an hour then peel off the film.
Coralfragger101,
IME, the affected area is going to have to be removed, a new bottom placed and new holes drilled. Boiling water is not hot enough, plastic needs to reach ~300F to be formable and placing that size patch will most likely cause the tube to craze (it is extruded)
Malifluous,
Yes, pour some resin down, let it harden, place your magnet, pour more resin over and around it.
CKreef,
IMO you're gonna have to do one wall at a time, use Weld-on 4 or equivalent. Glue one side, let it dry, flip it and glue the other side, let it dry, glue the bottom. If desired, glue gussets around the joints of areas that need to be water-tight or run a bead of Weld-on 40 instead of the gussets.
HTH,
James