Marsilea works well, but as stated, it can be a slow grower. It's about the only really dependable low light true carpet. Dwarf sag can work but no matter what you use, it can take time to spread.
The plants that come on mats or mesh are a good alternative, as they just need to be put in place and are already filled in. They can also be cut to fit. Dwarf pennywort and alot of the various mosses all can come this way. They are more costly but you get a lot of plants when you break down how much is grown in on the material.