The TW-16 is a large round hole adapter with a single width exterior porch and interior cling rod fits the tunnel for the TW-8, TW-8A, TW-8B, and TW-8C. It is for those who want interchangeability of the entrance holes, whether they suffer from SREH anxiety or are just trying to start a colony. The adapter is held onto the tunnel by 2 screws and just a little latex caulk for weatherproofing the seam where the tunnel and adapter meet. Once the female has built her nest, or laid eggs, one would simply remove the screws and put the crescent or WDC back onto the tunnel. The TW-16 sells for $4.50 each plus S&H.