This is the unit I got to stack these guys (although I like the rubber door stopper idea)
Halter LZ-309 Vented Metal Monitor Stand / Monitor Riser (Black) https://www.amazon.com/dp/B01G5ZJIRY
Halter LZ-309 Vented Metal Monitor Stand / Monitor Riser (Black) https://www.amazon.com/dp/B01G5ZJIRY