With two 100w heating elements in a 2' square, there is no cool zone. Shut one off immediately, make sure the other is as close to a corner as possible.
Most of the space should be cool, you just want a warm area large enough for the group to sleep in.
In small brooders, a brooder heating plate makes it much easier to provide adequate heat without risking overheating. And, it's efficient, the one I use outside in freezing cold weather only draws 50W. That's enough to warm the chicks and keep the nearby water from getting any ice even when it is in the teens.
And yes, pasted vents is a possibility, too, you need to clean them off if you find they have crusted feces stuck on their backside as that kills chicks. There is a small umbilical scab under the vent that you must be careful not to damage so please be attentive and gentle with this activity if you find it is necessary.