Using plastic for repairing, prototyping, etc. has always been a mystery to me until recently. I found a few good sites that show some techniques in plastic welding, bending, and vacuum forming. These may sound like high-tech methods that need special tools, but a soldering iron, oven and vacuum cleaner are as high-tech as the tools get.
Plastic welding is my favorite because of the simplicity and cool DIY possibilities. Using a soldering iron and oven paper, trash bags or other sheet plastic can be melted together to make things like kites, sails, or even solar
hot air balloons. Aleksi at Instructables
gives a four steps on this trick.
To shape acrylic/plexiglas, a heating element from an old toaster oven is used with boards and tin foil to melt specific lines for making bends. Sheldog23 at Austin PC Mods
built this melting tool and displays how to build it his modding site.
Vacuum forming uses #2 plastic
like one-gallon milk jugs; it won't shrink or shrivel like other plastics when heated. The method uses an oven to heat the plastic and a vacuum to pull it into place with a vacuum former made from a coffee can or something similar. Sheekgeek at Instructables.com gives a good quickie on vacuum forming