So got home from work early and started doing some more work. I decided instead of siliconing more chunks of cork to fill the gaps (didn't do it last night, ran out of silicone) and filling the void behind the bark with soil, I would use spray foam. I used small gap and crack filler since it doesn't expand as much and risk breaking glass. Once sprayed I put a generous layer of my soil mix on the exposed spray foam that wasn't going to be trimmed away, then I gently pushed it into the foam. I also laid a quick bead along the silicone on the sides of the bark and also laid soil over it, this was done to hide the silicone gap. That was left to cure and I moved on to my lid. I am an electrician so I had easy access to a diffuser for a fluorescent light fixture, they can be picked up at any hardware store for cheap. my lid has a screen top so it will support the diffuser, if making a lid from scratch get something sturdier. Diffusers are flimsy. I then measured the gaps I needed to cover twice and then marked out the cut with a sharpie and using a straight edge. I have tried different methods of cutting it and found the best is using a sharp utility knife and a straight edge for a guide edge. Watch your fingers. Once cut then file or sand the edge as it can be sharp and fit it on in.