Here are a couple pictures of my reamer with the guide in place. The guide is an approximately 1 inch long piece of 3/4 inch diameter Acetal (delrin). I turned the outside diameter to 0.740" to provide some working clearance in the 3/4 call barrel bore.
To make the guide I placed the delrin section into my collet chuck and drilled a 3/8 inch hole through the piece. I then placed the reamer in the tail stock, hand turned the lathe spindle and advanced the reamer into the 3/8 hole until I had almost penetrated the 3/4 delrin section.
As long as the guide does not bind in the call barrel the reamer will follow the hole in the call barrel without cutting into the guide. I would guess that a wooden guide would also work but I have not tried one. I got the delrin from ENCO. Look for Acetal rod. I hand ream the taper for the insert into call barrel and the drill chuck is merely a convenient way to hold the reamer.