I did something similar with the top on my flip-top cart. It is nice because it stops and you do not have to fiddle with lining up knobs or anything. I added a sliding piece side so that all 4 corners are resting on the sides. I also used window sash locks to make it all nice and tight. All 4 corners look like this.
--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.
