Biocube makes one, but the general consensus is that its not very good. Sapphire aquatics made one that would fit into the back of the biocube with top still on. The bad news is they just went out of business and replacement parts are scarce. Then theres the Aquatic Life Internal Mini Protein Skimmer 115. This one gets pretty good reviews for such a small skimmer. The downfall is I'm not sure it will fit in the back of the biocube and it definitely wont fit if you have the stock hood on without modifications.
On such a small tank you could get by on religious water changes. If you really want a skimmer you could either go open top with upgraded lighting which will allow for a HOB skimmer or drill the tank for a sump.