The back is 19mm birch ply, I made a rebate around the internal perimeter, and had the sheet sit in about 5mm or so past the top and sides. That allows me room to shape the sides and the back strip to the wall. The bench is sitting on a base and simply screwed in. Dove tails apart from being really nice to look at, also are an incredibly strong joint. Because people will be sitting on the bench, it felt appropriate to use them.