As you say, looks a lot like a self bailer that. Pretty much all the boats in the Hartleys range have transom drainage tubes like that and they work fine. We used to make transom flaps using thin clear plastic (think over-head-projector slides) with some insignia tape to act as the hinge along the top edge. Then to keep them shut drill a hole through each and put a bungee loop from one, into the boat around a hook and then back out to the other. The bungee generally stops the water getting in, but allows enough 'give' to let the water back out.
What in the hell is an elephants trunk? Is this like a wizards sleeve?