Yes, they are stackable:
"The Drum is manufactured from Linear Low Density Polyethylene through either rotational or blow moulding and has a high compatibility with foodstuffs and water. It's durability has been proven by Kaymac Rotomoulders via a 3 metre drop-test filled with water as well as extensive actual use in rural areas of South Africa and Angola.With the Q-drum, even a child can pull 50 litres of water over flat terrain for several kilometres without undue strain, and could shift the burden of water collection away from adult women to children and reduce the existing burden on the children at the same time - they play with it and love to fetch water."
Low Density Polyethylene does not seem to want to be used for fuel, so gas is probably out.
I can't find a way to BUY them...