If it’s a lawn type effect one is looking to achieve, then the material to use is ¼" flagstone. To add depth, wire mesh could be used in conjunction with the flagstone. One could cut the flagstone and wire mesh to the desired size and fit it accordingly. If it’s a rather large piece one is working with, tie the Riccia in sections instead of loading it on the object all at once.  If it’s driftwood one is covering with Riccia, do not cover the entire piece. Let some of the wood show. It looks more natural.  The only type of fertilization I recommend and use is Co2. There are numerous DIY gadgets but the best and less trouble-free is the pressurized system. Although the initial upfront cost is considerable, it is less troublesome in my opinion. A word of caution, despite the system you use, it would be wise to invest in a PH electronic monitoring device. This is specially so if you keep fish in a Co2 supplied tank. A good and safe PH level for both fish and Riccia is 6.8 . With the electronic monitor, if the PH should drop below this point, the Co2 would automatically be shut down.