A roller coaster sculpture or model can be built out of many things, but PVC pipe works really well. Because of the differences in heights of PVC pipe, you can have a roller coaster that is any size you would like it to be.
Draw up a plan for your roller coaster. Make it tall enough so that the car will travel for all of the track. Be sure that you design the correct number of hills and valleys, and that you have done the correct calculations to ensure that the car will travel through the entire track. If you aren't planning on having a car or aren't planning on actually having it run through the track, the exact specifications don't matter.
The PVC pipe should be used for the tall framing pieces as well as the cross-pieces. Decide on how many tall pieces you will need, keeping in mind that you need two of each height for each area. Then, decide on how many cross-pieces you will need.
Mark the measurements on the PVC pipe and cut it with a saw so that you have the correct number of tall pieces cross-pieces.
Build your frame, starting with the two tallest pieces. Stand them up and connect them at the top with the cross-pieces using the PVC pipe connectors. Continue with the other tall pieces and cross-pieces.
Connect the tall support pieces, once all the pairs are connected and standing up, to each other using the PVC pipe cross-pieces and the PVC pipe connectors.
Use your moldable plastic to create the model of the track. Remember that it should start at the top and then curve down to the bottom and back up again. If you mold it correctly, you should be able to set it on top of all of the tall pieces and cross-pieces.
Attach the plastic with glue where you need to.
Paint the PVC pipe and the plastic to get the look that you want.
Putting your roller coaster on a base and gluing it down will help it last longer.
Don't try to create a PVC pipe roller coaster that is big enough to ride on. It won't be sturdy enough for an actual person.