Thanks mrmum; that sounds like just what I am after. I hadn't thought of using the rudder pedals as an anchor point. Is that custom made or just a simple locking strap?
I might just have a play with a roof rack strap to see what I can rig up (probably a tenth of the price of a specialist aviation one!). It is really only the stabilator that needs fixing. The ailerons are kept neutral by the external control surface lock, and the rudder is linked to the nosewheel. If the wind is strong enough on the rudder to move the nosewheel, the plane is likely to be upside down two fields away anyway!