Chris,
Here are some photos of the parcel tray 9 years and 200 000 kms after completion (I.E. It has taken quite a beating in that time, so that's why it doesn't look the best). Bit hard to see the curves in the photos, it looks much curvier in person.
I started with a 3mm MDF board with holes cut for the speakers. Stapled the foam to the MDF, screwed the speaker grille on top of the foam where I wanted the speaker to go. Screwed a piece of MDF in the middle of the parcel shelf (where you see the indentation) and then squirted the expandable foam under the closed cell foam. Actually to tell the truth it wasn't even closed cell foam, it was just that white light density foam.
Just squirted as was required to get the shape. Left to dry, then removed the grilles and MDF cutout in the middle and covered with carpet. Dead easy, just takes some patience. If you put to much foam in, you can make a hole while it is wet and push the foam around, although be careful as it is very sticky and really only comes off while wet. Also when it is hard you could probably sand it and fiberglass it if you want it rock solid.