just make one. I did from a spare siding from the house or any plastic in that matter. Just get a hole cutter (2 size, one for center and outside diameter). First nalil the big piece of plastic you have to a 4x4 then cut the center leaving the DRILL hole in the 4x4 so when you cut the outside diameter, place the drill bit in the mark that it made from first cut so its centered, kabish ?
BTW if yours show the part of the hole, so will other parts will show the hole cause that piece is all the same size.