You only need to hit the 350 degrees when its first being cooked from the raw state, not hold your plastic at that temp. Absolutely let your plastic cool a hair to avoid the dents. And on re-heats or re-melts all you need do is heat until it can be stirred, usually around 330 degrees.
On the initial cook to convert the raw plastic I suggest slowing down and do the final heating to the 350 mark in short bursts [try 20 seconds] in the microwave stirring after each shot, but be certain you're using a probe type thermometer to read the plastic's temp after each shot in the microwave. If it comes out of the oven at , say, 340, stir it well and check again as the plastic can continue getting hotter even after its been removed from the microwave.