You can get a 50 litre Plastimo bladder for $90 at Whitworths ($120 for 100l). You can buy fuel bladders for not much more coin; I've seen off-road touring bikes carrying them in their panniers. Not sure I would want to carry fuel in the passenger compartment though.
These dudes have some good stuff. www.liquidcontainment.com.au, Motorbike fuel tanks, Petrol Bladder, Diesel Bladder, Av Gas bladder Tanks
Cheers,
Adam

