I haven't had my milk yet today.
I'm assuming you have seen the plans in the blue book? I thought about building one, but then remembered how I hate drilling holes so I just bought one. Steel isn't as cheap as it used to be.
There really isn't much to them. Width to give your bar a little wiggle room and height to fit under your ceiling. I think a lot of manufacturers give some or most of the dimensions. I could put a tape on mine if there's anything specific you need.