From there I was able to print out the cross-sections I needed, and transfer them to some 3mm balsa wood:

Which allowed me to create the skeleton structure of the part:

I then needed to in-fill the gaps, so I made some low density polyurethane foam using a 2-part pour in place system. With this being the product. I made 4 of these 'blocks'.

Then I used this foam to fill in the 'body' of my part:

Trimmed and sanded down to size, and voila!

From here, I filled and sealed the part with epoxy resin filled with microballoons:

Then used a stainless steel and fibreglass reinforced body filler to fill in the big gaps:

Flattened it down: 

Then used the same filler to cover the whole thing, so I could fill in any low spots in the foam:

Sanded it down again, then trial fitted it in the car:

Perfect fitment to the mm!