I made a shelf for my old 316i Touring, still got it somewhere.
I made it from 18mm MDF, it sat just below the load cover and it had 4 infintiy 6x9's in it.
You will notice where the load cover sits there is a recess, in this recess I sat a metal dowel (bit of studding would work) that reached accross the car, Perfect mounting/pivotpoint for a shelf. Onto this I mounted a piece of wood that fitted between the trim. Then on the underneeth of this you can attach the mdf sheet that is cut to fit close to the trims either side (you could use the boot carpet as a rough guide/template).
But you need to hold the shelf up somehow.
The level of the shelf will come accross the side access/storage pockets.
I made up two supports that consisted of 20mm square tubing with a triangular piece of metal welded to the top. This piece of metal was formed over the top of the doors to locate and keep the supports upright. The triangular pieces of metal were mounted near the top of the supports so while one face of the support is up against the storage door the other is sticking out 20mm into the boot compartment. If you judge correctly the mounting of the plates to the 20mm square supports, the shelf will sit on top of the supports. I inserted a plastic cap into each end of the support so it wouldn't mark the shelf or carpet.
I hope this has made some sense as it is quite hard to describe without pics.
If you have any q's, feel free to pm me
