Replying to Help me make my jib

I want to build some kind of a triangle rail, not sure what it's called. Triangle shape with the rail on top and sides slanted down. Pretty sure there's been a thread on making one of these before, but I can't find it. Right now I'm thinking having it 8 feet long and a foot and a half or two feet high. I've got 90 feet in 2x4s for the frame, and plywood to cover the sides along with a pvc tube for the rail itself. My question is what angle should I make the sides at? I want it to make them not shallow enough that the rail will be 4 feet wide, but I don't want it to be so steep that you wont be able to play around on the side of the rail. Also if there are any other obvious red flags that I'll come across when making this, it'd be great to know.