I don't want to get into a fuel consumption debate, but I think its good to be conservative in your calculations. Better to have too much fuel than too little. However, I doubt very much you'll average over 20km/l for the trip. Whilst I myself get as little as 10km/l on sand, this is not 'generally' the case and I found 14-15km/l is a more reasonable figure for consumption/distance calcs.

But a lot of this depends on whether you can actually ride sand or not (no offense meant). I've seen people who manage quite well on an unloaded bike become totally unstuck on a loaded/heavy machine. And if you're going to paddle your bike through the sandy sections, then expect to use a lot more fuel. The good thing is that most of the track is actually as wide as a road and not just two lines in the dirt/sand.

But getting back to you original question- how to carry extra fuel? Without the obvious solution (and expense) of an aftermarket large fuel tank, most people prefer fuel bladders which can be rolled up and stored when not needed. Other popular options include Rotopax (or the cheaper knock-off versions). And then there is also the cheap option of using steel / or plastic jerry cans and abandoning them on the way as you're finished with them. (Someone else will pick them up and use them, so don;t be worried about leaving junk inn the bush)

As said before, the ADV website is full of people with different options for unsupported fuel options- e.g. look at all the Simpson crossings and how they do it.

Good luck

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Squily