I have thought of doing this before. What would be wrong with using the glue, as long as you let it dry before introducing the animals? I see no harm in this...you drink, make your pasta water , and bath from water pipes that have this glue on them everyday?
The drawback to this design, in my mind -and you touched on it, is how/where do you attach the heat tape and cover for the top? Do you stick the heat tape directly onto the tub? This would require glue though? How do you keep the top on and the animals in? Also, it seems that you would lose alot of heat unless you boxed in the sides, but how would you attach then?
I am not trashing your idea, I am hoping to pick some new ones up for myself. I actually think PVC pipe is a GREAT low-cost, exremely sturdy, easy to wok with medium.
By the way, for belly heat, I believe 3" FlexxWatt ran all the way down one edge of the tub is how most folks do it. I use the 11" stuff, but I have back heat which is much less efficient in transferring heat.
Please keep us posted, and share your ideas on this one, or some drawings or plans if you have them and want to share.
~Alan