The thick PVC used as shower pan liners is glued using a solvent type glue. I have been told you can use regular PVC pipe cement but I used the solvent cement specifically intended for the use. I suspect it is the same stuff dyed a different color. Pretty much any place that sells the plastic liner material will have the right cement. Many places will have a one-page instruction sheet available for free that gives detailed instructions on how to cut and weld the material. I got mine from Home Depot and the cement and instruction sheet were all on one shelf.

Typically you cut to shape, coat both sides of any joint with a layer of solvent cement, press into position and clamp firmly. I used two pieces of 2by4 and a large C-clamp. After a day I unclamped it and the weld is almost as strong as the material itself. I've been told, like with PVC pipes, the material doesn't need to be clamped as long as you can hold it together for a minute or two.

Simply folding and using removable clamps to hold the shape, instead of cutting and welding it to shape like I did, should be quicker and easier. Clothespins might work and might do double duty holding the plastic in shape and securing it withing the cage. The plastic wants to lay flat so I would plan on allowing it to run up the sides three or four inches.

To give you some idea of what I'm talking about. Don't let teh price scare you. They are selling a complete roll that is 5' wide and 40' long.

http://plumbing.hardwarestore.com/51-290-misc-repair/pvc-shower-pan-liner-637284.aspx#features