Just a thought; although it won't help anyone who has already built their oven. I'm sure everyone strives to pour their pad as level as possible. It just makes sense. Everything after the pour is that much easier to build. But mabey before stacking all the blocks a sill seal could be placed under the first row. Alls that neds to be done is set it on the pad and start stacking. The weight of the blocks and concrete fill is more than enough weight to compress the seal and make the wood storage water tight. As far as the opening is concerned; a threshold would suffice. Something as simple as white oak. White has a much tighter cellular structure than red and less prone to rot. Three or four coats of a marine poly and held in place with a good construction adhesive.
