Yeah ya don't want soot flavored ice cream, so an extension might be in order. A coffee can (soup can in this case) would work fine, flappers are just fun to watch at idle for everyone. Will you run a muffler? I've never heard mine without a waterlift muffler. The only sound you hear outside the hull is bub-bub-bub-ploosh-bub-bub-bub-ploosh. And you have to be standing on the same side of the boat as the outlet to hear that. Inside the salon is a different story (with the ER hatches removed a demented monkey in a 55 gallon drum with a couple ball peen hammers would be quieter)
What does your engine use for a water pump? Since mine is raw water cooled it uses a seawater pump only.