What I did was goto the craft store and pick uip a grid of that plastic stuff for needlepoint. It is flexible and easily cut. You can connect them together with the wire from a twistie tie and it barely shows The ones I got were 10 inches by 13 inches, maybe there are larger ones if you need bigger, or just connect them as I mentioned.
They are all white, so seeing the mantis when they crawl on it is going to be very easy