I almost hate to post this because the skimmer that I found to be really great currently isn't available. I have a 24g aquapod and sapphire aquatics out of Houston made an awesome skimmer for it. They were making skimmers that fit each specific nano model. This skimmer works very well. You might post for a used one.

The other great option that I have found is the Tunze skimmer. I don't have experience with this one but I have heard great things about it from my own research. Most people put the sapphire and the Tunze at the same level of quality, and I think they are currently the best available for a nano tank.

It does cost more than others out there though.

http://www.aquacave.com/detail.aspx?ID=1436#

Someone really good at acrylic work could model a skimmer after the Sapphire come really close.