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I used a piece of regular glass from the local hardware store and they cut it to 10"x10" for free. Then I used a glass cutter like the one below to score the glass on a diagonal at each corner to form an isosceles triangle with the two edges of the same length being about half an inch (IIRC). Then, I had my kid hold down the glass using a straightedge at the scoring line which was also aligned with the sharp edge of a table below. Then, I took a pair of vise-grips and snapped off the corner. Remember to wear safety glasses.

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