I know this is not what your asking but I made a 200lph canister for one of my Fighter (Betta) tanks. I used a tiny pump inside a plastic container with a clip on lid and filled it with media and it works a treat. Heres a link to 'Moodys' filter which is very similar.
I've got an Eheim 2211 on my 10 gallon. It's not exactly small, but it's got quite a low gph rate that makes it really good for smaller aquariums, if that's what you're aiming for.
the smallest external cannister is an
Eheim classic 2011.
you do however have to order it direct from Eheim as they
don't supply any shop with it.
It is not on their website either so you have to phone/email and ask