I have 3 PVC rails. They work fine and they slide well. There really isn't a material that's as cheap and durable as PVC, but if you wanted to make a box or something you could use HDPE. Using metal is hard unless you know how to weld and you have an up-to-date tetanus shot.
If you really wanted to step your park up, you can give these guys a call. Their stuff is very pricy but it is quality.