I think John Eacott is on the money with this one.
I had a conversation a few years ago with a very experienced pilot about this and his view was that on a private flight you could carry (I think) up to 3 x 20lt containers, properly secured etc.
The main point made was that they had to be UN approved and therefore NOT the metal ones. In fact also many of the plastic ones were also no good.
The only ones he suggested were ok were the black ones with the yellow funnel, two openings at the top and so on. He said these were the only ones (at that time) that complied with all the regs, and had the correct stamps on them.