I moistened the log and flattened it overnight wrapped in a towel with weights on it (about 40 pounds). It didn't hold its shape without the weights once dry, but in the process the back of the bark cracked in the right places so that I could flatten it dry with a little force.
I turned the aquarium so that the side that was going to be the back was laying flat on a work table then laid down a layer of silicone. After this was done, I put the cork in, flattened it with the weights (under a towel to keep from damaging the wood), and siliconed the edges. After 24 hours the bark was flat as a sheet of plywood and well adhered to the aquarium. I checked for signs of disbonding after 48 hours and didn't find any.
After more than a week, there were no signs of disbonding or condensation.
Marcos