Fresh cork sheets in thicknesses of less than 3/16ths and less should be bendable to fit a Harmon type mute. Otherwise you'll have to steam it (DO NOT SOAK!!!!). First, make template. Still, there was an article about such with a foam material found in craft stores at much lower cost. Cork sheets are harder to find albeit some craft stores and hardware stores do carry such and I believe so do tech suppliers. My own experience has been only to utilize a trumpet Harmon on a 'bone and this was done with a custom made metal sleeve insert that was then mandrelled by tech to the 'bone contour and then foam added vis I could remove sleeve and continue to use mute for trumpet. I really detest all mutes!