You should be able to buy freeze plugs (they are inserted in auto heads to seal the water passages-any auto parts store) .I believe you need them sized 1 1/4 inches,(can't remember! but they come in all sizes) they can be driven in from the inlet side of the muffler about half way down,with a wooden dowel,(their shaped like a bottle cap ,insert domed end first), forming a baffle.

The stock H-D baffles is just a round flat solid  plate ,people knock them out or drill them ,either because they think it will make their bike faster or they like noise . The freeze plug idea isn't mine it has been used by a few members on the site ,it's cheap ,and it works .I  think Dave came up with the idea. when you look in the muffler you should see an inner pipe drilled with holes all around and nearly the whole length , theobject is to drive the plug so that is centered in that section of holes, so the exhaust gases are forced to go out the first half of the holes around your new baffle and return through the holes and exit the muffler , if you can look through the muffler the stock baffle is damaged or missing .

Rotorhead,Yes  let's lie and say "more power",and louder when you sell the mufflers,  when you can buy a freeze plug for 2-3 bucks and a length of dowel or pipe for a couple more , and drive it down the muffler , a job an 8 year old could to in ten minutes with a bit of adult guidance .Science project? If this sounds hard I hope you have someone to work on your bike.

It is a 1-1/4" diameter.

Take a flashlight and look down the muffler.  You should see a series of holes in the inner pipe, then there will be a gap without holes, then the holes will start again.  If I remember correctly - there are more holes on one side of the gap than on the other side.  You may also see the remnants of the steel plug that the factory put in.....before the previous owner drilled a hole or knocked it out.

You want to install the freeze plug from the engine (inlet) side, and it will go in easier if you put the flat part of the freeze plug in first and have the formed side on the backside.  Drive it down to the area between the front and rear set of holes.  (This forces the exhaust to go through the holes and into the outer shell of the muffler).

I may try this too.  My (wife's) Savage blew the guts (I guess it was the baffle) out of the HD muffler going down the highway.  I was following her and saw something kinda orange fly out of the pipe - It was at night and looked cool but I was like 'WTF was THAT?'
Now the bike is loud as nuts and has got a serious low to midrange stumble.  I kinda tuned it out using the idle mix screw, but now it backfires on decel.

Thought we were stuck with it like this because I didn;t want to get into re welding or replacing the whole exhaust - this happened like 2 years ago.

I am glad to see there may be a way to fix this.  What keeps the freeze plug in?  Friction?

Gotcha,
I will shine a light in there and see what is going on.

James,That jar lid might not hold up to the heat over time,and may bend going in ,you should be able to use a freeze plug as they come in many sizes and are made of much thicker metal. I'd really go that way if you want a low cost, strong ,long lasting fix.