Whiteflash....scroll down this page a bit(training parameter for prowler) and you'll find someone made a blueprint for one. I made one of my own and just guesstamated. One note though, if you will be transporting it in an SUV, make sure it's narrow enough to fit. I made sure I measured my wifes Hyundi width before I started cutting up the steel for my own.
The pic of mine is in My New Baby thread at the end to maybe give you more ideas. I used round steel tube leftover from work stuff. Took about six hours, with beer breaks of course. If I made another one I could do it quiker. A pro could do it in less time.