I built my own triangular corner treatments. I just copied the Michael Green model, and did the other two types as well at the same time. The Jon Risch information that I picked up along the way, among others, REALLY helped a lot. The end product uses burlap (most acoustically transparent - you don't want the material reflecting), stuffed with long hair carded wool (best stuffing sonically). All of the materials cost me about $55, plus the sweat equity.
The improvements were not subtle. Highly recommended.
The improvements were not subtle. Highly recommended.