An OHG must be tested by the manufacturer to the point of failure to satisfy OSHA/ANSI requirements and then you can build a copy of the original. This proceedure is very expensive and explains why the price of these items is so high. If you design and install it you are liabe for any safety defects in the product. I would agree that the idea sounds like a time saver but the reality is very dangerous. If you can't park closer or disability of the worker is a problem maybe you should check in to a small personell cart that you could load and unload with your truck mounted lift.

I am not a lawyer or an employee of a government agency but I have been in the business long enough to see good intentions turn in to disaster.