Another pic

I don't see that you have much room for a muffler.  Installing a dirt bike type silencer on the end of the existing pipe would look a bit weird...and it really wouldn't do much to make things any less noisy.  A big 4 stroke needs a baffle to smooth out the large pressure pulses to cut down on noise.

The best chance you have for fitting a muffler would be to install a Supertrapp muffler at the end of the pipe.

http://www.ebay.com/itm/SuperTrapp-1-5-Clamp-On-Silencer-Body-without-Core-31...  

If that doesn't work I believe you could fabricate a muffler in the middle of the pipe that has a narrow body - but a bit of height to it.  Some early trials bikes had mufflers that were similar.

I designed and ordered cabinets for an efficiency apartment. I didn't think to find the sink first. I hadda modify the thing and settle for a shallower sink,
Moral of the story is scope a project end to end. Hope you can work it out.

I think you could build a muffler very easily if you can weld, cut sheet metal and drill holes.

Cut your existing pipe off just a bit before the shock, then fabricate another pipe that will rest on the top or bottom of that pipe and exit ou the back like your current one.....and overlap these pipes as far as you can to look visually pleasing.  You can weld these pipes together lengthwise along the joint...a few good welds to hold it in place not a full length weld.  Then take the pipe off the bike and begin drilling holes in each pipe - a bunch of 1/4" holes spaced 1/2" apart or somethings similar....the holes should be very close to the joint as the next step is to cut sheet metal to cover the sides and keep the exhaust inside your new muffler (The DYNA muffler that works so well on your bike only has about a dozen holes this size.)  Then fabricate the flat side pieces and end caps....you can slope or curve the end caps to make them more visually appealing.  Get the thing welded up, weld on a mounting tab or two to attach the muffler to the bike, and you will most likely end up having to remove the shock to get it mounted if you make the tail pipe go inside the shock (over the belt drive).  You can weld on nuts to the side so you can fabricate a heat shield to keep from burning your leg.

If you really want to get fancy....you could make the muffler from stainless and have a slip on clamp fitting to your current header.

You could make something that looks a bit like this....although the inlet/outlet pipe would be offset in the method I suggested.

Pic

Ended up with a biltwell bullet cannon and fabbed up my own adapter, mount and brace. Sounds good looks good happy.

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