I don't know it this will help you, but I was recently trying to mellow down the tone of my overly-bright kona. I bought a cheap (Oz$6) plastic version of the lute hole, which had a piece of black sponge backing, so it acted pretty much like a solid soundhole plug. I gradually cut pieces out of the centre of the backing until I got the degree of muting/mellowing that suited me. I similar approach might work for you.
I bought mine at the local music store, and can't find it with Google, but it looks something like this, in cheap faux wood with a black foam backing:
http://www.musiciansfriend.com/acces...edback-control