I've been making crown boards with bee-escapes for use in my Paradise poly hives. I bought a load of 2mm correx, that I cut and glue together, 'warp & weft.' This strengthens it. I then glue an inner bit, so the board fits more precisely, without having to guess it, when there are thousands of bees charging about! Next I cut the long 'rounded holes' through the first two layers, then trace and cut the square hole through the third layer. A touch of glue on the porter bee escapes and then they are pushed into the hole. I'm hoping these this will be more effective than the wooden ones that were built for ordinary national hives.