Trust me on this one, you want to attach the cubby top now, and not later If you are attaching with screws or nails, use 2” fasteners and glue. For pocket hole users, drill 3/4” pocket holes around all sides of the cubby top (one of the 15 3/4” x 20 1/2” pieces). Then drill 3/4” pocket holes on the 15 3/4” sides of the remaining 20 1/2” long pieces. 3 per board side is plenty - one on each end and one in the middle. NOTE: If you inset the top, keep top pocket holes down enough for the inset. Then attach cubby top to sides, keeping all pocket holes to inside.