The first thing to do was rub down and polish the fuel filler recess, this was done by hand due to the confines of it. The centre of the mounting surface was found and an 76mm hole was cut with a holesaw.
The Aston filler cap comes in two parts, the threaded flange and the cap itself. The two parts were separated and the flange bolt holes drilled to 5mm (these were already pre-marked on the underside) and countersunk. With the flange held in position on the car I then drilled through for one of the mounting holes, then bolted it in place to drill the rest. The flange was then removed again to make the gasket.
I used countersunk Allen head bolts, but you could use button heads.
I bought a large roll of 1mm thick rubber gasket material off ebay along with a tool to cut circles (basically a compass with a blade attached) and some punches for the bolt holes. I then made my gasket at 135mm diameter with a 76mm diameter hole in the middle, remember to cut the outer dimension first otherwise you'll have no centre left for your compass point! With the ring made I placed it on the car under the flange and marked bolt holes which were then punched out. I used an M6 punch just to give a bit of wiggle room for the bolts.
The flange and gasket were then bolted to the car and the cap screwed on. Initially it didn't tighten up in the right place. The cap comes with several shims, the thicker red ones are 1mm think and the thinner blue ones 0.4mm thick. After some experimentation I needed the 3 thick ones to get the cap to sit in the desired position.
Bolted on to car, looking lovely (although I may still get it powder coated black yet....)