Military fuel tank seal help

I've got the original twin military fuel tanks in my lightweight under the seats, There is a thin seal fits into the filler flange on top of the tank that the lid seats on to, is this available anywhere?
Have checked Lrseries website and came up with part 551743 now unavailable.
I have seen these in cork and rubber on other tanks
any ideas chaps?

Try blanchards as they have a lot of military parts.
Otherwise....
Large Cork Sheet Gasket Material 500x1000x1.5mm Thick | eBay

Could this be it from Theex-mod
91110527 - Gasket - 66MM OD x 50MM ID x 3MM THK - 91110527
searched under fuel tanks 713 options
Or this one
http://www.thexmod.com/item_detail.asp?id=629
Not sure if this will fit under seat tank
http://www.thexmod.com/item_detail.asp?id=1127

Thanks for the response, but it's none of the above. The filler cap goes straight on to the tank under the seat, no filler neck. I would love to cut my own but it is a large diameter and only 2-3mm in thickness and width, so near impossible to cut. Will carry on with my quest!

Is there a lip for it to sit around?
Could you use a suitable O'ring?