The manual neglects to point this out, but you definitely need to round over the outside of the top of the box (3/8″). Fiberglass cloth has to go around this bend and it won’t lie properly unless you round the edge. This is confirmed in the picture on the bottom right of page 351.
Also do not use a router to round the front of the box, which is slanted outward. The tool guide will dig into the wood. I used the router to round the back and sides and came just slightly around the corner. That way I was able to use the router’s rounding as a guide for how much to round the front edge with a sander. I also beveled the front edge of the lid to match the slant in the box. This is not called for but I decided it would look better. So, I couldn’t use the router to round the front of the lid either (neither top nor bottom rounding of the lid front–both had to be done with a sander).