Okay pics attached. Here is my thought process - I did not want to risk dumping metal shavings into the muffler, and I didn't want to try to uncrimp/braze it back together. I also did not want a totally free flowing muffler, some backpressure is needed. I think I found a pretty good setup.
You can drill a hole (or cut a slot) in the two tubes that slide out of the muffler. These tubes are somewhat directional, so mark their rotation orientation with a sharpie. I specifically wanted the holes that I drilled oriented where exhaust has to flow "around" to the sides of the tube. IE I didn't want a straight shot for exhaust gasses. I added a couple of arrows to show where the holes are oriented. I also wanted the holes near the opening of the tube, away from the heads of the exhaust bolts.
So, that gets exhaust gasses through the muff. Next step was the cover. I bent open the slot a little bit, but ultimately ended up shimming it open with a couple of washers. This part I am not super happy with. I might end up making a shim for this, not sure yet. I left the spark screen on.
Edit: Also. You can do this entire process without removing the muff from the saw. Just do one tube at a time. That way you don't have to mess with re aligning the gasket and plate behind the muffler. And - I drill a hole all the way through each tube (so there are two holes in each tube). In hindsight I would've probably started with one hole in each tube. Simply because I don't know that there is any need in allowing THAT much more airflow.
