Well done binky is looks like you've thought up a brilliant way of over coming a horses fear of plastic bags there!
My pony crispin used to hate donkeys but then the lady in a stable near mine bought one as company for her ex-race horse and cos we ride past it so often Crispin has completly got over his fear - although the first time it brayed at him was a 'interesting' experience! but thats another story!