Mt Bob
Minister of Fire
Harman sells one with oak for their stoves.I recently installed an duravent for 4" pipe and 3" oak in a 7 1/2 hole.A thimble is very easy to change.Icel makes a nice one,duravent has 2 kinds of different sizes.Separate OAK - It is supposed to be installed lower than the pellet venting. Since you already have the pellet venting and the thimble installed, the other suggestions are a little late