I just ordered 2 media reactors from davids tanks n innovations and he can make you a hang on holder for them and can make them any size you want, He does quality work at VERY cheap prices, I paid 70 dollars shipped for 2 12 inch tall by 4 inch diameter reactors and a hanging holder for them
these aren't really external hang on style but are small....maybe they could work for you. These were prototypes I saw on reefbuilder a few months ago, not sure if they are out yet.