I have thought about this, and I know what I need, but not what it is called or where to get it.
That aside my idea was some stainless sockets flush in the deck, and 3 or 4 levers on the removable seat which would go through the flange that meets the deck. This would be a half inch (ish) bar with a T piece at the end which would locate in the socket and be tightened with a quarter turn. You see a larger version of this holding containers on lorries.
The nearest equivalent I can name is a Dzus fastener, but these are way too flimsy.
I hope you understand this, I find it a bit difficult to explain clearly.