Fuel tank neck seal

[colonel mustard]

I have started to fit the twin fuel tanks, the Lh tank has fitted well and is secured in location. The rh has been a bit of a pain!
I have cut a boot board square and using this as a gig to make sure the tanks are parallel to each other. We I come to line the tank up and put the rubber neck seal on, it appears the hole cut in the car is slightly larger than the LH side, creating a small gap with zero overlap of the seal (Photo below, close up of the underside of the filler cap)
Has anyone else had this issue, if so are there alternative neck seals that someone has sourced?
Any advice welcomed.
Thanks