Thanks everybody for the numerous suggestions. I have decided on a mongrel solution garnered from all the replies. A box on either side of the jacked up car. A plank or piece of 4"X4" laid across the two boxes and the underbody thingy is balanced on this with the front end tucked under the front wing. you can lie under this and take your time and perhaps also saying a prayer to the car god. His co-operation is always welcome. An awful thought --- maybe he's female or worse still gender fluid!
Fred