Any decent model shop that does aircraft should have the stuff you require.
The flying lads use thick walled large bore silicon tubing for their exhaust couplings. Take your brass Huco coupling with you and just try them and get one that is a tight fit on the coupling. A 1ft length should last you years. You will find that it will not slip if you get a good fit.
Huco couplings, engineering wise, are made to terrible standards, and most have some sort of built in wobble. Putting that tubing between the two ends absorbs all that, and things go very quiet, very quickly. In fact, in the end, I stopped buying the couplings and made my own better quality versions, still with the tube between though.
The local model boat club have been using this method for many many years, in fact I think it is one of the first things new members are shown how to do when they have a noisy boat.
Bogs