John,
We clean our catch on board more often than not. Most of our take is 15 to 30lb yellowtail, dorado and tuna. They are immediately gilled and bled in the ice box (this one bled before getting to the box).
Most guys have a bait tank about 30" tall so we lay a 2'x3' board centered atop the bait tank. It stashes nicely under the gunwale cap. There is some spillage but because the fish have bled out in the box its not too bad. Board has a frame on bottom side to keep from sliding off the bait tank and some routed grooves to catch some of the mess. Always a hose with a pressure nozzle on board to wash away blood.
Of course I've never seen a boat as nice as yours but I empathize with your desire to keep it that way.
I your case I might be inclined to cut fish with board resting on cap and draining outward. Todd and everyone else makes cleaning stations that fit into rod holders so you might look into that. Or make your own.
Here's a pretty nice set up from one of the local guys. Scroll down to thread #8
http://www.bdoutdoors.com/forums/boa...ing-board.html