[Blue Angel]

Ever thought of using some type of modelling clay to make the shape of the insert, then use that to make a mold? You could produce a light weight, tight fitting piece from fiberglass or something similar.

This might be a bit more work than doing a one-off from rigid foam insulation, but the easy sculpting nature of modelling clay might produce a nicer final product. Not to mention, if you screw up with clay you just add a bit of material back on and re-work.

This is how most surfaces are defined in the studio when styling a car, but instead of pulling molds the body is digitized and modeled in CAD.