Was thinking that the furniture bolt would be inserted from outside the top. A threaded cam lock on the inside to cinch it up tight against whatever gasket material you use. The flat bolt head could be recessed and bondo'd so that it is not seen at all from the outside.

Another idea - set up on the inside a series of cabinet latches like those used on small tool boxes. You can even get the stretch rubber latches like you see on tilt front truck front ends. They also come available in a cam lock format. Drop on the top, snap the latches and away you go.