[Turk]

Check with your local body shop supply house.
There is a product known in the industry as "FART BLOCK".
It is dark gray, black and looks like styrafoam.
It crumbles, and it is sacrificial and will conform to the shape of the surface that is being sanded.

It disintegrates and in the process emits a rather foul smell.
Hence the name.
You'll need a bunch of it.

TURK