hey guys need heellp!
basically ive included pics of what im doing with the wide arch to ilustrate what i mean as well.

now the thing is when the door is closed you cant see the gap but when the door is opened theres a massive gap because obviously i have a 5 door which makes life a bit harder when trying to do rear wide arch.

im planning to use expandable foam to make a sort of platform in order filler over.

BUT NOWS MY REAL PROBLEM!

if i use normal filler or even fiberglass it will eventually crack after a few hard slams of the door.

what im thinking to do is use a flexible PLASTIC filler which should in theory withstand the viabrations of the doors being slammed and is also easily painted over.
this type of filler is also very commonly used in bumper repairs so im guessing its VERY VERY flexible which will be a bonus in regards to withstanding the door being slammed...

this was just a guess from what ive read up..

can anyone shed any light on the topic or recommend any other materials to use....
i need a material that's easy to sand, flexible in order to withstand the vibrations from the door being slammed and also can be painted over...