I've working up a drawing of a strap design that will enable the spring to work to tension the strap but will have a felt backed leather pad behind to protect the bonnet
Perhaps something akin to the way a NATO watch strap works?
That was exactly what I was thinking although the 'under strap' wouldn't be attached at one side so that the spring could expand.
2008 XXVII Platform, Bugatti Blue Roadster 4 Seater