I've used plywood of 4mm thickness. Since my glass is 20 by 30 cm, I cut the base to the same width as the glass, and that is 30cm. The length is a bit bigger, in this case 22cm so the camera support will fit. For the sides I cut two more pieces of 22cm width and 16cm height. The triangles are equilater with a side of 12 cm. Cut also some small parts for reinforcement. The two triangles with 45 degrees angles will support the glass.