I have invented the whiffle ball! :-)
This fits into the transom drains for my boat's bilge and splashwell to keep leaves and other debris from clogging them. The extension can be added to the drain tube so that it extends past the outside of the transom to avoid stains from water running down.
This has been working well for me for three months now. I have a similar design for the cockpit sole limber holes at http://www.thingiverse.com/thing:444685, and for the bed of my pickup truck at http://www.thingiverse.com/thing:464492.
This is printed in HIPS without support. The spherical part goes on the bed and the tube prints last. Print one or more of transom_extension.stl and glue them together to add length to the tube.