Thanks for your reply Bogertophis. The interior diameter is 3 1/2 inches by 3 1/4 inches. It is attached thru the walls with 3 inch PVC pipe. The tunnel has a hinged bottom for ease of cleaning and it is attached using a French cleat so I can easily remove it from the wall if needed. I am planning on using corrugated plastic to fashion a tray in the tunnel for ease of cleaning. From what I've read, corrugated plastic is pet friendly. I still have a question about off-gassing. In the Fir catio, Truesdail (my BP) will have four chambers with varying temperatures. Do you think the sealed Fir wood will still be safe once the wood is introduced to a heat source?