For horn length x = at throat to at mouth
Take a length of y mm diameter braided cable sleeving
Fill with stone aggregate of particle diameter > max sleeving wall gap
Wrap around horn at x
By varying y one can adjust damping at different points along the length of the horn, if that turns out to be beneficial. Another advantage is that because the sand is not bonded one might get better damping than with most other materials.
Apologies for DIY flavour, but what, I wonder, do people make of the following technique for damping relatively thin-walled large horns, in pseudocode...