12. Take measurements to double-check the glass lid size. Ideally, the lid will fit in so it comes to rest about 1" above the top opening of the box. As shown here, a ¼ × 17 × 17" piece of tempered glass fits just right. Be sure to order glass with polished edges. You can also just use a clear plastic oven bag instead of the glass. Either will trap heat and speed up the cooking.