i just made a quick one of these for a Mk1.

Had to get the plastic side pieces off a member here

I then cut a sheet of Plyboard to size, cut out holes for speakers.
I used a length of aluminum with holes cut in it and a few nuts and bolts to hold the shelf to the carpet "hinge".

The i chucked a rug over it all and bam. rear speakers on the cheap.

Though i will remake it later on with a thicker board, i can see mine breaking in a few months . or ill add a thick piece of wood as a brace. either way i have the whole lot there, so making a new one will be easy