I have a question on how to attach the lid? I tried to use 1/2" hex bolt to attach the lid from the top through the OSB. With the washers on it sticks up too high. My thought was I can just drill holes in the cement board for the hex bolt head and tile would go over the top. But now I think it's up too high and want to swap them out. I tried to counter sink on a test piece but the OSB lost integrity so I didn't do it. suggestions?