I found some cloudy white plastic tubing of the right size in a scrap pile in my shop. It is some tubing that I used in fixing up a neighbors "high boy", which is a funny looking rig that is used to spray crops.
I put that on and it works much better. However, it took awhile this morning for the temp to stabilize. I theorize that this hard plastic line was cold and took awhile to stabilize at a temperature near the desired cabin temperature. I'm going to try again tonight with some pipe insulation like engatwork used. I believe the thermal properties will be more conducive to this application.
Thanks for the help,