Make the outflow to the bog lower than you think it needs to be, and make the sides all around the bog taller than you think they need to be. As the plants and roots grow they act as dams and raise the water level on all sides of the bog. (I know this since my bogs have overflowed on the back and sides too many times, and it recurs as the plants and roots grow and the gravel raises) Since your liner is what it is, maybe lower the gravel by removing some so there is more buffer for water to build up and not over flow.