For the SUPERTWIN tank it cost a bunch.
i don't know if you saw it, but the aluminium guy rebuild a tank that as nearly the same shape than the supertwin original fairing.
if you look on the details of the pictures you will see that the fairing has been cut just before the seat.
So it is an amazing work to do.
This guy is the one who made the guzzilla aluminium swing arm for Dynotec.
About a closed tank that looks like the original oem, i have bought for my "standard daytona rs" (not for long) a special alumium tank made buy a german guy for 485 eur with airbox closed space . (not the same guy than for the supertwin).
i found it cheap for a complete tank modified.
Perhaps he could do the v11 tank but i think he doesn't alrady do it.
So from a 17;5/18 L tank i go up to a 21.5/22 L tank.
In the desert area of alps or central france, or a Sunday at lunch time it could be usefull
I will collect his contact informations, and will give it to you.