I am not sure how this should be done with any accuracy, but luckily enough, I was not too far out and did not have to hack away at too much fiberglass.
I will try and show the fuel filler cap mechanism in a later post, which is still work in progress. In essence it is a mechanism from a Vauxhall Corsa, which I have then cut and epoxied with the aid of some 3D printed reinforcements.
After test fitting the fuel tank, I decided to seal it with Slosh fuel tank filler to make sure it was well and truly water (fuel) tight.