Lazyman commented about 14 hours ago 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.
Oh, I see. You have a wide sliding piece that part of the platform sandwich. That's clever. I'm going to start without any clamping like you've done with your sash clamps. But I have some ideas in mind for doing that should I find them necessary.