What material muffler should I use?

Second picture shows a plywood template I made to get the right location and the right cut angle on the rubber correct.  Shows it after I cut the tube since I didn't think of taking a picture before that.  I took a piece of 6" wide plywood and, using a hole saw slightly bigger than the silicon tubes, cut out an oval that fit neatly over the tube when the ply would be at the angle that's parallel with the bottom of the boat.  Then I put the tube on the exhaust outlet, slide the plywood up the tube till the plywood was against the bottom of the boat and ran a marker around the top of the ply against the tube. Took the tube off the boat and cut it with a sharp kitchen knife and then put the tube back on and clamped it tight.  End result is a neat cut line that is even with the bottom of the boat.

I took the boat for a quick spin before installing the muffler tubes and then after installing them.  The boat is noticeably quieter, tough to give a sound reduction level but I could easily hear the difference.  Top end didn't seem to change, was still 43 MPH on the air guides.  

I did drill the 3/16" hole in the top just above the bend.  I also had external rubber flaps which i removed before installation, I didn't notice any issue when backing the boat off the lift.  Exhaust was bubbling out just like normal so I don't expect any problems backing up with no flappers.

 its effective, inexpensive, non permanent

we're start/stop in front of the same houses each morning, so we got ahead of any issues

I prefer fiber mufflers like centek, like Rob sez,  i go further by remove flappers  add a 45 degree reinforced silicon elbow,  cut it flush to the running surface. almost as quiet as more advanced 2-into-one systems or fea without the drag in the water.

let me know if you want to know more details and particulars

https://centekmarine.com/solutions/propulsion-wet-exhaust/

You can find mufflers from Centek all over the www

JQ