The easiest way to attach the top plywood panel is to add a rabbet to the frame. That is a little recess that the panel can drop into. I used a trim router and made very small passes until I had the desired depth and width. The corners will be round, so you either need to make the plywood panel round, or make the corners square. I used a chisel to make square corners. This looks more difficult as it is. Just take your time and don´t worrie. Its hard to mess this up. Scoring the wood with a utility knife helps.