I make my own, just use a small one and then take it out to the size you require, I use a countersink bit, and twiddle it by hand, so I can carefully get the element size I want, and measure it with a vernier. With plastic ones by coming from both sides you can vary the thickness as well.
I also do the same with metal elements, and just a dust of matt black spray after.
Walther KK500 Alutec expert special - Barnard .223 "wilde" in a Walther KK500 Alutec stock, mmm...tasty!! - Keppeler 6 mmBR with Walther grip and wood! I may be a Walther-phile?