you can take a piece of slate or even round stone and first add a small layer for riccia and top it with the mesh, tie it on the bottom with either fishing line or something else.. and eventually you can have a lil carpet bush. With wood its the same thing, just use fishing line to have the mesh semi tight on the wood.
BUTTT you will have to make sure you trim the riccia no matter if its on wood or stone, if it gets 2 tall, the lower part will die off and it will break off the stone/wood and it will be a mess.
From my experience, riccia can be a very nice addition to the tank. It adds alot of bright green color and looks "nice" but eventually after trimming it you will see that its a painful plant to have.