I've made two mufflers, both are absolutely silent. No need to weld, solder is fine and copper tubing works great. Find a shop where you can buy a short length (18 inches) of two inch copper pipe and a couple end caps. Probably enough details in my photo album to show you how to make it. Basically a Cherry Bomb muffler.
I made a video to show how quiet the muffler is, but haven't been able to figure out how to post videos on the forum. At a C Dory get together a member walked down the dock and asked me to turn it on so that the could hear it. It was already running and pointed right at them.
Some of the little heaters have a one inch combustion air intake hose. They make a silencer for that that works and will be needed if you want total silence. Actually, total silence is difficult because the dosing pump ticks. I wrapped mine in pipe insulation and suspended away from the bulkhead. Can still hear it when sleeping, but it's not a problem.[/list]