I think I heard Joshua P. Warren call it that. Basically 6 square mirrors assembled together into a cube, with all the mirrors facing inside. People who have made these have reported demonic sounds coming from them, growling, stuff like that.
I have a suspicion some of these things have an affinity for corners. Many people have experienced those hateful evil presences in the top corners of their bedrooms. These mirror cubes have plenty of corners should that make a difference.
Anyways, seems to me these might make great focal points to assist in summoning. There’s lots of horrible videos on youtube about these things.