Jack / Nevermore,
For most of the winter I just left the valve open, so when it rains heavily, the water just drains out of the bottom. Next winter I may build a cover, similar to a cold frame to keep off the excess moisture as well as to jumpstart the plants in the spring. As you know this has been one cold and wet spring in the PNW and I wish I had more plants under plastic / glass.
Nevermore - the bog is complete and I used a bulk head. I need to sit down and finish the original post!