I never noticed till you pointed it out that it is hard to find smaller diesel cans. I have a couple Wavian NATO 5 gallon metal gas cans. One came mounted on my antique truck when I bought it. I liked it well enough that I bought another to hold gas for my small engines. They are the supplier of military spec cans to NATO. They much more solidly built than the imitation cans. I checked, and they do make a 2.5 gallon diesel can. Not cheap, but Wavian makes some good stuff:
Buy Wavian Gas Can Yellow 10 Liter at Swisslink.
wavianusa.com