Well my brother made mine for me, and basically he built a box unit to hold the sub and amps which fitted quite snugly in the boot, and the sub box sank into the wheel well (spare wheel removed) It all depends on how you want you ICE set out as to how you make the box.
Then he got a bit of 4mm MDF after using polystyrene (paper will work) just to get a template of the boot. He then cut out the holes where the amps and sub will show through. Then covered that in foam, then the silk.
As it is on 4mm MDF it is flexible so we then just plonked the top bit into the boot. As the box underneath is a snug fit it didnt require any fixing down, but thats up to you if you want to do that.
Here is the finished result. If you want to travel to Norwich, im sure my brother wouldnt mind making you one for a bit of cash and the cost of materials. Wouldnt cost anywhere near as much as going to somewhere professional.
Mine isnt finished though (the amps need to be raised up to be level with the top)
http://erc.qmuc.ac.uk/cliosport/gallery/full/1094919555__CIMG0002.jpg