I made the parcel shelf out of 5mm mdf. The shelf in there when I bought it was 10mm laminated particle board. Particle board is a terrible material, do not use it. Go any bigger than 5mm thick in mdf and you won't be able to bolt the rear seat belts back how they came out. I believe 5mm mdf is plenty strong enough, I'm only going to have 6 or 6.5" speakers in it.
I made my new shelf using the old one as a template, then got my dad to cut it with a jigsaw. If you don't have an old one to go off it'll be a little bit more tricky, but I'm sure you can do it. I'd recommend cutting it to the basic shape, being conservative and keeping it bigger. Then just keep test fitting, taking little bits off at a time...the biggest worry you'll have is the protrusion on the passenger side. Be careful as you'll probably scratch this trim.
I haven't done anything else with mine yet, it's still just bare wood atm. I plan to grab some carpet from Clarke Rubber and cover it myself.
I've only got one pic of it, I can grab some more if you want.
Hope this helps mate,
Sian