I love the hobbit themed scape.
From the site:
Silica sand, lava rock, some grey stones, small battens and driftwood
Lighting: 2*15W T8
CO2: Pressurized CO2 for first 3 months then removed."
For the mound, I would use a hairnet to wrap moss around the lava rock. I did this with Christmas moss and it looks great.
In regards to the tree, I think you just use thread to wrap the Fissidens around the branches. I think the secret is to get a piece of wood with lots of branching so that the canopy looks full when it grows in. Unfortunately I am just guessing since I have not done the tree thing yet.
Do you think there will be enough room for the betta to swim? How will you handle algae? Perhaps you could add an amano shrimp. Please share a journal of your project as I'm interested in how it will turn out. Good luck.