I would put in first the heating cable (with suckers, I suppose) to get the right spacing between the loops. Then a thin layer of gravel, as support for the cables, and to make sure they don't touch the bottom glass. Then the laterite, then the gravel.
Assuming that it gets warm where you live, you would need some kind of temperature controller that turns off the cables when the water reaches a certain temp. Not sure if that was included in your setup.