If you have been following along, you will recall that the bog has already undergone three iterations. One with the ill fated 215g tank, then two fiberglassed versions for the 240g. While the fiberglassed one looked good, every time we turned on the water it leaked. This, despite having fully sealed just the wood frame with fiberglass resin, then adding foam and supports, covering it in fiberglass cloth and sealing that. Every time I found a leak, I sealed it with silicone, let it cure, then tried again and it would find somewhere else to leak. Finally, I gave in and we tore it apart. It turns out that the water that had gotten between the layers of fiberglass was quite nasty as you can see here, and was apparently the source of a gnat infestation in the loft.
After much effort pulling/prying, some Clorox to kill the nasties, then a thorough pass with a wet/dry vac, we reduced it to its original frame once again. I had added expanding foam between it and the tank and between it and the wall in an effort to stop leaks, so we weren't able to remove it easily with the tank in place and we weren't about to take that apart!
A key in the new version was that the water should have no choice but to run back to the tank. This was accomplished by filling the inset portion with play sand, sloping it toward the tank. The idea was that we could carve into it to form channels, but we didn't end up doing that.
Next we covered the wall and the bog with pond liner
Folding and trimming it until it formed leak-proof sides and would funnel all water back into the tank.