The best way for me to use moss as a foreground is coconut shells.
They can be placed anywhere and also be removed to trim.
I buy the coconuts from the supermarket at .50 cents each.
I use a table saw to cut it in half, also sometime I use a hammer and a flat screwdriver. I gut out the meat with a screwdriver and use it for Chicken, potato and onion curry over rice. hmmm
I use a plier to shape the edges and to pull off the bark. The next step is to drill holes all over the shell so you can use fishing line to thread the moss on and also make it easy to tie. The other reason I drill a lot of holes for water flow, so there will be no dead spots where the shell covers the substrate. Now I soak the shells to remove the tanner.
As a aquascaper I always keep extra shells around. Besides moss, I use it for riccia and java ferns. Sometime it's not easy to tie on plants to a driftwood depending on the odd shape. I just tie it on a shell then place it on the wood. Over time the roots will attach on and can be removed too.
I would not use cotton thread since later on the moss will detach due to not enough light. The more dense it gets from trimming overtime will only kill off the the first layer of moss that was first attach to.
Light fishing line works the best for me in the long run.
I would only consider using cotton line for Anubias nana and ferns watch work well.
Yes, hairnet riccia coconut shells work well. The only problem is it will float when the riccia pearl. The bottom of the shell need to be anchor with lead tape or gravel in the hairnet.
Yes slate will work too, but coconut shells are easier to work with and easier to tie with pre-drill holes.