Rather than putting a plate on top in the microwave, I have resorted to using a lightly dampened paper towel, gets the job done just as well and one less dish for me to clean.
FWIW the USDA says plain paper towels are fine to wrap food while microwaving [1]. But I agree with the notion of wanting to minimize exposing food to toxins. I try to limit my microwave usage all together and prefer the oven for reheating food.
