8147
This is nicely sized, though somewhat bigger than what you have for halfpipes currently. The only problems with this piece is that it's butt ugly and you can't select it by clicking on it, but the elements list is easy enough to use. I made a half pipe with this in about 10 minutes. The fence is see-though, but there are also solid beams you can line up easily.
To make things much easier be sure to use CTRL + F9 to copy the object in the same location as well as SHIFT + MOUSE SCROLL to rotate objects in large increments (rotate 180 degrees to effectively mirror the fence on the other side and keep the same slope. Very useful after finishing one side of the halfpipe). That should help you line everything up easily.