Top and base have 2 coats of primer and after a brainstorm on a fresh thread we have a fastening method for the top.
No screws or heavy catches , no rattling and no weight win win !
So I think DannyC had the brainwave to use Velcro and I just perverted it to the cause by using double sided Velcro.
Test pieces the upper one [ lid/top ] has a strip of heavy duty cardboard front and rear and a slot cut for the Velcro
Lower one [ base/fairing ] has a piece of heavy duty cardboard glued both sides and 2 slots cut for Velcro
The cardboard resists the Velcro pulling through the corrugated sides.
Velcro is threaded through slot in top one a round back where it is stuck to itself and a tail is left hanging down
Velcro is threaded through both slots in base piece and stuck to itself around the back.
Viola as the French say ...
One neat fastening no weight or rattle and only Velcro pulling against Velcro.
Downside ? there are 2 small slot's in the base that need sealing against water ingress , compared to it's bulk I don't think they will be a problem.
Implement on the real thing then I can lash some paint on.
Paul