There are multiple styles for coping. Many people use pvc pipes, but I find that proper steel works much better. If you're willing to spend maybe like 5-10 bucks more per pipe, and put a bit more time and effort into it, use steel.
As for mounting height, either mount flush with the topsheet, or a fraction of an inch higher. The problem with higher is that when the metal is sticky, you don't slide. If you can get fairly precise, mount as flush as you can because if you use a good topsheet material (HDPE), it slides really well and is very dense.
As for styles of coping, some people i know have used pvc pipes, some have used metal pipes, and some have used angle iron. I would mostly reccomend metal pipes, simply because they will last longer. The point of coping is to stop your edges from digging into the plastic topsheet, so using pvc kindof defeats the purpose of coping because your edges can dig into the plastic. Angle iron is also a good bet because it sits in well with the box frame and is much easier to mount onto the box than a rounded pipe. It's up to you though.