u can built it out of anything
people have use kegs
propane/gas bottle
ammo boxs
iv used a fire extinguisher
i know someone who used a lunch box
pretty much any thing up to a cereal box
if u want to make it easy
look for something that is already water prof (so the fuel wont leak out)
make sure the fuel wont eat it (some plastics melt in petrol)
find something that has screw fittings on it so its easy to attach fuel lines
preferable to have 2 holes in it
one at the bottom and one at the top
one to let the fuel out one to let the air in
but u can make any thing meet these points with some fabrication