Hi Dave
As the space in a 40 is so limited the best option is to make a door to cover the opening to the door storage and to just line the inside of the door with carpet or the material of your choice so as to utilise all the available space.
A fibreglass insert would not allow you to use all the space that could be available to you.
Done this way you will be amazed at just how much usable space there is inside the door.
Dimi