I started by making a 0.063" spacer which I tapered at both ends to fit in the space between the SS firewall and the fueslage skin. Just when it fitted perfectly, I realised this wasn't going to work - DOH.
The tapered part takes up the space alright, but the flange would be further from the edge of the skin at that point, meaning I would have to compensate with extra fibre and resin on the inside of the cowl.