To be completely honest, they are roaches and will be around long after we are gone... I wouldn't put to much thought into getting the temps just perfect. The truth of the matter is that nature is not perfect and that temps flucuate quite often. Another thing to note is that they are a tropical species which would range somewhere around 70-85... think hawaii weather, not arizona.
but in any case, i have used heat cable stuck to sheet metal, an UTH stuck to sheet metal, and finally heat tape which seems to work the best to heat a large number of tubs... by large number think around 30.
-Kyle