Best method to plug outboard well

hi -

finally decided to just close the outboard well on my international folkboat and put the engine on a garelick transom mount. apparently this solution is the one most international folkboats with a well end up with, because of the unhappiness of dragging an engine in the well while sailing, the wash coming up into the cockpit when heeled over, etc.

i suppose i could just epoxy a piece of plywood of the right shape - but i'm guessing there'd be a better way to go about this? what material should i use? worth it to make it more easily reversible? elegant ideas for attaching the plug into the hole?

the hole is about the size of a rounded off legal sized piece of paper.

thanks for any ideas.