Besides the brute force way. I want to learn a more efficient route.

I am trying to draw this Speaker Icon shape starting with a square. I was able to chamfer one set of angles (the 20 degree) but when I try to go for the second 60 degree chamfer i get the "Chamfer could not be added because the specified distance would eliminate one of the elements" Is there a reverse chamfer? Or how would you go about this?