An idea I've tried for a washboard setup using larger alloy channel with 3mm ply sandwiched between two 13mm cedar washboards top and bottom.
The inner bottom and top boards slide down the channel and the outside top board overlaps the inside bottom board. The outer boards screw onto the inner boards and sit flush against the companionway bulkhead giving a strong resistance from the cockpit side.
If the pressure comes from the cabin side it means you've already sunk and don't need washboards any more.
any comments/criticism welcome except personal ones ...(I know I'm already mad).
Some photos:-