It is 12cm wide, (that's cen-ti-me-ters, it's part of the metric system...also called S.i. or "system international") and about 3.5 cm high at the edges, and another 1.5 cm to the bottom. I don't think the dimensions are critical at all...you just need a big hole in there to spill off pressure. I used a dremel disk to cut out the rough shape, then a small dremel mill to smooth it out, then 300 grit sandpaper to finish. Total time was about 30 minutes.