How about this train of thought...
Something like one of those collapsible cups but in a bigger format to fit the back of the vehicle. It could be made out of coroplast quite easily fer shape and low weight. Attach it to the back of the vehicle, pull it out and lock it in place when you need the aero capabilities and unlock/collapse it back down when you don't need it.
Do you really need to go to the very edge to git all of the aero effects? Can't you just kinda step it down from the edges to git some effect? Sort of a step/chop of the extension...a compromise? Just a thought...