I used a piece of plastic board, about 5mm think, cut to size, and with 2 strings attached and hung over the gunwale. From memory I had to shop around a bit until I found something I was convinced wouldn’t be absorbent, I may have got it from an art & craft shop in the end, it’s a sort of closed-foam mounting board - sorry I can’t be more specific. Photo attached - Windermere on one side, Loch Lomond on the other!
Also photo attached of locker catches, mine seem to be fitted a bit squint, they do the job and can be locked, but there may be better alternatives. The yard also supply a kit to seal the lockers lids if not done already, for me this provided a bit of resistance for the catches and made them more secure.