In our dart frog vivs we used the cat litter method for a back wall, but trying to do that in yours would be walking backwards. I have seen folks use a variety of things for the back wall, but some things can catch on your darts small toes and cause injury (they are surprisingly great climbers, but terrible swimmers and will hurt themselves on anything possible). Maybe silicone some precut corkboard in place? I'd certainly suggest a glass top (silicone it in place) to maintain humidity and keep in the fruit flies. They even escape our silicone lids somehow and we had a single tank that was not done and it drove us insane. Darts can eat quite a few so there are always errant flies making their way out. Viv looks great and I really like the layout and design. maybe try some Tillandsia mounted on that wood.
"In the land of the blind, the one-eyed man is king" Desiderius Erasmus