Making a Flexible Cowling - DIY Vac Form?

[dr_gn]

Guys,

I've had to bogde together a new cowling for my 1:72 Specuial Hobby Curtiss Hawk becasue the original was too wide at the rear. There were two cowlings in the kit, the 'wrong' one for my version was a good fit at the rear, but wrong at the front. I cut the rear of this one and stuck it to the front of the original, and filled and blended the join with Milliput. It needs some cooling gills adding as a strip, and re-scribing, but the shape I'm now happy with:

Trouble is, becasue the corrected profile tapers to the rear, I can't fit the engine in without filing the cylinder heads off. If I do this, then by the time it's pushed into the correct position in the wider, slightly bulged part of the cowling, there will be a gap between the heads and the inner cowl. Idealy if I could make a more flexible cowling using the modded one as a pattern/former then I could fit the engine by springing the cowling around it.

I thought about Vac-forming, but using my modded cowling this would give one that was too thick by the thickness of the plastic used.

Any ideas?