I wouldn't even want to begin the try and design that mechanism that has all the corect angles and such so that it workd properly. Why re-invent the wheel. I'd just get one out of a wrecking yard and install/adapt it to fit in your bus.
Yea go for the wrecking yard, you can choose from two different styles for cheap. Got mine for $20. I took 1 look at the various metal bars and hinge points on that thing and decided right then that fabricating one would be nearly insane unless I was an engineer with a metal shop and a ton of time on my hands.