Here's an idea for a very cheap way to carry 10L jerry cans which I used on a honda dominator but may work on other bikes. I replaced one of the pillion footpeg mounting bolts with a ~5 inch bolt (bolt only had 1 inch of thread - by putting a nut on this when it was tightened it fully secured the footpeg bracket). I could then rest the 10L jerycan between the bolt and footpeg and secure it with straps. All this (including straps) cost ~£3 - slightly cheaper than a rack but longterm durability is unknown. I used them predominantly with an empty jerrycan (therefore light) so may be a useful idea for people not requiring extra fuel all the time.