To be honest, you don't really need one. Salt is cheap to do water changes on a tank that small. Carbon can also be cheap to just absorb the organics and not skim them. A skimmer might be more trouble than it is worth.
Coralife makes an inexpensive skimmer just for a 29 gal. BioCube and I believe should sale for around $40. But I do agree with jda123 above, why use one.
Ive got the Coralife skimmer for the BC29 Id be willing to part with, thing worked great, I just upgraded to the Tunze 9002 when I installed the 7gal sump.
i am using cpr aquatic sr3 . it's specifically made to fit 29 gallon bio cube in the middle part .you can only use it if you have a lot of rocks in your tank . because you have to get rid of bio balls and drop in filter