Do not place heat cable inside the enclosure it gets quite hot too hot to be safe inside. I would not recommend a heater inside where a animal can contact it for anything but a bump, and even then I would be leery of anything that was hotter than 100ºF. I have cable I use on slab sided pvcx and a roto mounded vision and just use al. tape and also have used electrical tape both have worked perfectly. Have you cleaned the outside with alcohol to remove any surface oils sometimes used when the material is cut. Did you try good quality tape or cheap tape there is quite a difference between the cheap and the more costly.