For those who have a mother, sister, aunt, or are female
One thing to note in the transition from glass to plastic is that the glass conducts the heat far more effectively than the plastic and this may well be what accounts for the difference - that coupled with the fact that there is now more volume which needs heating. Is the rubbermaud directly on the heat pad and what is the heat pad set to? If you bump it does the surface (of the substrate) become too hot? I have mine set on 1/2' "rails" (front/back) to keep enough ventilation under it but have had to bump a couple the past couple days due to an overall drop in temps in our area.