Rather than wonder about the toxicity of your reef epoxy (one's sold for the aquarium trade are generally harmless), just use plastic Zip ties. Certainly cheap and easy to get. To avoid dead spots in the back of my tank, I place powerheads in the back to blow water around. Either make sure they are somewhere you can get to them easily (gotta clean em once in a while), or make sure they have a good prefiltering system. I have a small Eheim buried in the back of my tank, get back there and clean it once or twice a year. Been working great for me for 3 years now.