Plug one outlet then couple the other one with some Teflon tubing to a cheap ($3 or so) lawn mower muffler. Mount the lawn mower muffler wherever and however it makes sense. Another option is to couple using copper water pipe and fittings. You can slit the end of the pipe and clamp it to the muffler outlet/inlets. Use high temp exhaust sealant if you need it. You will still need to support the extra muffler somehow.