The TW-6 is a 4" diameter tunnel that is 2 1/2" long. It has a crescent entrance adapter glued to it. The adapter plate is glued so that it cannot be changed. This is the tunnel adapter to use if you do not suffer from SREH anxiety. The adapter plate has a single width exterior porch with an inside cling rod. The bottom of the tunnel acts like an interior porch. There is a drainage hole drilled in the bottom near the front and a small dam glued to the back of the tunnel. Both help to keep water from entering the nesting area. The tunnel is good for any natural, or Supergourd. The tunnel is good for adding extra depth to a gourd, especially if your gourds are on the small side. The tunnel is mounted by tracing a line around the tunnel when it is placed against the gourd. Then, take a sabre saw, and cut the line. You want as tight a fit as you can get. Insert the tunnel as far as you want. You can barely put in it, you can put in part way in, or you can put it all the way in. Also, I am willing to make the tunnel longer if you feel it is needed. The TW-6 tunnel sells for $6.00 plus S&H.