right the way i made mine.
take the original shelf and place on top of a sheet of MDF.think i used 8 or 10mm
trace out the shape of the original shelf.
cut out the MDF,check the fit of the shape and trim as required. to get mine sitting flat at the rear window side i only had a tab of mdf on each side near the back of the car rather than the whole length of the side supports(does that make sense?)
now if you want an OEM look then cut battons of MDF and hot glue them to the shelf in the shape of the original shelf.
now using a sharpknife cut out the metal support pole from the original shelf. fasten this to the underside of the MDF shelf.you could use brackets or as i did hot glue(do you see how much i like hot glue?

)
cut you holes for your chosen rear speakers and fit the speakers.
cover with acoustic cloth/carpet. can be stapled or as i did hot glued
remeber to tension the cloth for a nice clean finish.
can be done in a couple of hours easily. i did mine with trips up to the flat(top floor)as i have no access to power in the car park