Its always a fun process. I like coir mat because its long lasting relative to something like moss. Of course the LONGEST lasting materials are cork bark, which is sustainable, but has skyrocket in cost, and treefern, which is NOT a sustainable resource and has become rare. And large pieces of ornamental driftwood, but something of the size needed would be really hard to get at a reasonable cost.
Another option is a bit more adventurous......constructing a wire enclosure frame and filling it with Great Stuff foam, letting it harden, carving/sanding it to look like a tree trunk, then coating it with a plant friendly base that is used in terrarium/vivarium builds and dusting it with an overlay of moss and EcoEarth (ground coconut husk). This is the most common way people build artificial trees.
