Storing the doors behind the seats was a good idea, but because the gap was very narrow I found that the back panel was getting rather scuffed.
and as it showed through white on black it was very obvious and let the car down.
So the first thing to do was to try and move the seats. I had tried before and failed, as they had been in a fixed position for 5 years, but after a lot of 'gentle' persuasion I managed to bring the backs up a bit and give some extra clearance for storing the doors.
Having fixed the reason for the damage, I then just built a vinyl-covered addon from hardboard and glued it on.
Looks very smart :-)