Years ago there use to be a company in germany that made extra large tanks for cx's. It was probably due to the lack of availability of fuel and want to get somewhere without running out of gas.
Here's a listing and a fun price of $495 euros.
21 Liter-Tank für CX500C - Auch mit Lackierung!
Die Letzten! Produktion eingestellt!
495Euro
That's one hell of a price for this big boy.
Well since it cant be bought anymore LRCXed Larry offered to try to make one.
Here's a mock up of what it should look like.
The top is a before and the after is hopefully what it looks like finished.
Click the image below to see the progress of the build.