Chaeto would be your best bet. Rather than pay 15-20 bucks for a ball way too big for you to use, try to find a local hobbyist who will give you a small amount. There are small clip on lights that would work ok. While some people recommend 24/7 lighting, there is some evidence that this isn't very good for the plants and algae in the long run. The bonus of opposite photoperiod is evening out Ph swings. Though with such a small fuge, it may not make a huge difference.
Just because the Ph swing argument confused me for a while, I'll explain it. When plants and algae are actively photosynthesizing (lights on), they are converting CO2 to O2. CO2, in high levels, can bind to H2O and lower Ph. By having photosynthesis occurring somewhere at all times (12 in the DT, 12 in the fuge), CO2 doesn't have any period of time in which to build up excessively.
Again, an HOB fuge isn't going to affect that much, but just throwing the info out there.