For ease of water changes, trimming, etc... I would tie the Fissidens to small lava rock or small pieces of wood (stuff with lots of pores, nothing real smooth) and space them out evenly. then let them do their thing and form the ground cover.
I'd go for individual pieces myself instead of a large mesh screen. Easier to spot treat sections. If one piece gets some hair algae in it, just take out that one piece of rock, snip and your set. If one side is getting a little busy, pull out that side and trim.
Pulling out a large chunk of moss to trim might seem like a timesaver but it isn't in my book. Too hard to handle, creates too much disturbance when putting it in or pulling out.