I personally use steel tubing for a pipe and use a hole saw for the hole.
The mufflers on husky’s stock are pretty thick gauge and hold up to a hole saw fairly well. However if you want to do it a cleaner way get a diamond hole saw instead and take it slow. Less aggressive and no chance that it will oscillate.
Pick a hole saw size that is the same size or just smaller than the steel pipe OD you wish to use. If it happens to be smaller then slowly dremel the circumference until your pipe fits snug.
Brass with bronze flux coated rods, you will need oxy acetylene or a B tank in order to get it hot enough. Cherry red and it will melt the bronze rod. practice on old steel stock of similar gauge until you get the hang of it.
maybe your done the best way to remove the left over flux is with a wire drill brush wheel, a stiff one will make short work of the glazing.
finally sand with whatever you wish to make it look pretty, or go straight to the degreased (brake clean and paper towel.
paint with engine heat paint of the Desired colour, choose one that is good for 450-700 degrees that doesn’t require a primer. couple coats and let dry for 12-24 hours.
put her in the vice and apply a heat gun right to the flange end and start pumping hot air through. When it smokes it’s starting to cure, continue for a couple more minutes then stop.
you will end up with a muffler that looks pretty decent with a nice round much larger port than stock that will be as strong as the muffler itself. The paint will hold up to the abuse that a saw receives.