I've looked at this and there is no legal way to mount them without a cage bar at the apropriate height. As PZG302 said, there is a limit of degrees for mounting them. The reason they don't let you mount them to the floor as per Scando's post is that in the event of an accident the low mounted straps will contract your back and can potantially leave you in a wheel chair.
I hope you're still not driving with this setup