They are mostly used inside fake waterfalls where they are completely hidden. I did a little DIY project to try and keep one in my newt build. I used a a 2" piece of PVC with a notch cut out for the cord. I kryloned it black and then hot glued a thick plastic screen over the top to keep the critters out. Also kryloned black. Trick is to only hot glue one edge so that you can lift up the screen like a flap to access the fogger. It worked well.
I hvae some of that weird screen left. I have no idea where to find it. It works perfect for this application and I hvae never seen it since. I found it in my classroom one day in the deep dark recesses of the science stock room. Who knows.........its probably 20 years old. I can easily put a piece in an envelope and send it to you if you want. Just pm me if your interested. good luck and keep us all posted!!
Something similar that I have done to keep critters out of waterfalls/homemade filters is to use a piece of pvc like you mentioned, and instead of gluing the screen/mesh directly to it, get a pvc coupling, and glue the screen to that, it should slip-fit over the other piece of pvc, making a lid that's accessible, yet keeps the critters out. As to plastic mesh, I've used the plastic canvas that they sell in craft stores, I think it's made of nylon and typically white/natural colored, though I have seen black (and other colors) numerous times. pretty inexpensive and pretty useful.