Interlocking plastic, rubber, or foam product that can be cut to size?
I believe that Matt's suggestion of what is called plastic mesh is likely to be the best find.



The image above shows a typical product known as 7 mesh plastic canvas, for 7 holes per inch and lists as being 13.5 by 10.5 inches without a thickness reference. I've seen this material in the past and found it to be about 2 mm, perhaps slightly thicker.

Another site has similar material, a wide selection, including some 5 mesh plastic canvas. That particular product must have rather large openings, allowing for an approximate 5 mm between holes! As such, it's likely to be stiffer as well.