Welcome to the bowing problem I had. The heater actually starts to fluctuate as the temperature drops and raises during prints. I first attempted to solve the problem by using a large 3mm borosilicate glass plate which did help with the problem but I also had cold spots towards the edges. Currently I am running a 300x300mm heater mat glued to a .1" piece of aluminum and the 3mm borosilicate glass on top of that. This resolved my z banding problem, using aluminum standoffs helped with the ringing as well. Heat up times are longer but the glass gets hot evenly and stays very hot. If I were to do it again I would look into a heater mat powered by AC instead of DC for faster heatup times and less stress on the power supply.