If you plan to use a finish slab of wood on your window seats, you will need to build some type of “sub-base” to attach the wood to. This is also true for granite slabs or other finish seats. We build ours from two 3/4″ slabs of scrap plywood screwed together. We cut them about 3″ narrower than the seat and never wider than the distance from the window to the edge of the bales so an apron can be attached to hide them.
We add 3 1/2″ long screws from the top down which anchor the bases to the bales and then add the second layer of plywood to the first with 1″ screws. The protruding screws really grab the bales well, so it is best to build them in place so you don’t have to mess with getting the base in the right place once all the screws are sticking out. This system allows us to attach a finish window seat to the sub base with ease and anchors the whole assembly to the wall. If you are using plaster mesh, you can add that over the top of the base as well which really holds everything in place!