Reply to "Stopping rain etc getting in through rear deck lid aperture"

I have to say I think you are tackling the problem from the wrong angle. The opening is recessed under the decklid and the only way to get real water ingress is if you are parked with the rear facing into the wind driven rain. If you are moving then it's not an issue, at least from rain entering the decklid opening perspective.

Virtually all the water, grime, dirt etc. is sucked up from under the car as road spray (my car is white and the decklid gets grimy from road spray exiting the decklid opening) the only cure to that is a belly pan, even then it's almost impossible to truly seal. IMO the best you can do is use some water repellent products in the engine bay that make it easier to wash (with water!)and keep clean. Get out and make the best of the British weather then still smiling wash her down when you get home

I note you have a delta wing already, is it a later or wide body car? Do you have a sugar scoop spoiler that sits close to the top of the decklid?