Macs Products produce a muffler that should meet your requirements. It is a one piece bolt on unit.

Muffler #6400
Adapter #6608

Rossi also offers a bolt on muffler that is very quiet, though not the most durable. The outer shell is crimped on and will vibrate loose with use. Some have had the joint welded after which the muffler seems to hold up.

Macs and Rossi both produce headers and muffled tuned pipes if you are looking to maximize the R.45's power. The Macs Quiet Pipe is the lowest noise version.

RR