Is it possible to unfasten and lift the box slightly to gain access for thin fingers?
No, not without removing the engine management system to get at a screw under it :-( I'm reluctant to do that on a new car. I hate to think what Morgan would say in the event of a warranty claim

I've managed to get a strip of P section weather proofing sealer down the side of the box. A gallon of water poured through the heater inlet resulted in NO WATER INSIDE

Now I just need some rain

Ian