This suggestion may not be for the faint of heart, but might be a permanent fix.
Get a bronze nipple or hose nipple that will fit your rubber exhaust hose,
Cut off the nipple damaged inlet nipple from your muffler,
Using a hole saw about 1/8 inch smaller than the new nipple threads, cut a co-centric hole in your muffler where the old pipe exited.
Get a piece of high density plastic that is rated for high temps, check out USPlastics.com, maybe PTFE, (maybe try stock nummber 45450) and cut a plate that is larger than your new bronze nipple, large enuf to allow room to bolt this right to your muffler, around the edges of the plate.
Maybe there is a plastics expert out there who can help.
Cut a hole the perfect size to thread,(tap) your new bronze nipple into this plate.
Now I don't see where you said if the muffler is square or round. If the entrance side is flat that makes things easier. If curved you will have to shape the back of the plate to lay against the curved face of the muffler.
Once you have the plate all shaped and fit, clamp it over the old bored out hole. The hole in the muffle should still be too small.
Drill and bolt this plate to the muffler using high temp gasket maker. Don't skimp on bolts. I would use Stainless and large washers, spreading the load. It may be a bugger to get the nuts on the inside of the tanks, so either get extras, or use sheet metal screws, tightening carefully, use lots, also SS.
Once the plate is bolted you can re drill the hole, making the hole in the muffler the proper size.
(the undersized hole just might have made it easier to line the plate up for bolting, but I think its important to get the nipple threads to catch both the plate and the original muffler)
Screw in the new nipple and go cruising!
But first, send me the left over PTFE so I can fix my muffler!
Come to think of it, if your muffler has flat sides, you could just bolt on a floor flange and save a lot of trouble.
Comments welcome. Work in progress.