Speedway motors polished gas cap 4-5/8
http://www.ebay.com/itm/Speedway-Mot...BVvLj4&vxp=mtr
This is a lot larger and cheaper than anything else I could find, it also looks perfect.
f1 by
jaguartvr, on Flickr
To fit it I cut the mounting panel in half to enlarge it and then in half again.
cap2 by
jaguartvr, on Flickr
Here is a picture of it mounted with the rear clam off.
fuel7 by
jaguartvr, on Flickr
I did find that using the concertina effect fuel filler pipe from CBS restricted the fule flow so I bought 500mm of straight pipe designed for a boat. Fits perfectly and the fuel flow is much better, you still have to fill up with the trigger pulled half way but if you look at the picture on Smash's thread youl will see it is much better.
I used rivnuts to provide tapped holes to mount the flange as getting nuts on the mounting screws was just about impossible. Don't forget to provide a drainage hole otherwise it will fill up with water.
cap4 by
jaguartvr, on Flickr
I don't really want anyone fitting the 4-5/8ths filler cap, at the moment I can comfortably say I have the biggest on the forum and would like to keep it that way!