Marimos lie flat but they never truly attach to anything. So your best bet would be to spread them out and glue them to mesh/rocks. They grow really slowly. They also tend to collect and show a LOT of debris. They look great in pics but in reality they're a pain in the butt in my experience.
Xmas moss will grow a lot faster but will also be a LOT taller/bushier than marimos. It will only work well as a carpet in a bigger tank IMO. Mini x-mass moss is tall enough to make up the midground in my 3.4 gallon nano.
You could also try fissidens/mini fissidens on mesh.