When flushing my outboard in the driveway I tend to shut it down way to soon so as not to offend the neighbors. The noise really reverberates throughout the neighborhood. Came across some old posts on another site where a guy made up his own muffler. I tried it with some modifications. using common plumbing supplies and some stuff I had lying around the house. It worked great. Cut the noise better than half.
First, measure the size of your prop hub. Then get a flexible rubber coupling. Mine was 4". Got the parts from Lowes. Next a 4" PVC clean out fitting with plug. Also used a 3 1/2" round electrical cover, cut out a gasket from an old rubber backed stair tread. A 4" galvanized 3/8" bolt, washers, nut and a spring from an old thermostat kit.
Drill 5 - 3/8" holes in the PVC plug and 1-3/8 hole in the center of the elec. cover. Invert the plug and screw into the clean out fitting. Bolt goes thru center of plug then thru rubber gasket, then elec. cover, then spring, washer and nut. Spring and gasket are there to relieve excess back pressure in case of a backfire. (see pics) This set up also provides needed back pressure for a more thorough flush.
