The plastic is sort-of medium thickness, but pretty tough, and has one-inch conductivity of => 20 mega-ohms (good enough for most of my stuff). Dialectric breakdown of the washers is unknown – but I’m only using them for the 5 to 12 VDC type stuff in my SoC collection. I probably won’t use it for any higher voltages. This is not advice for anybody else to copy – but is just what I did today to save a trip to the home improvement store. The nice thing is the selection of colors, and the fact that I can shape them however I like! I’ll have green ones soon. I can stack as many of them as I like, depending on my application. They really don’t HAVE to be round.
They can be square, like me.
Note: this is NOT advice, and not meant for duplication by others, and not necessarily OSHA approved. Neither is my “manufacturing” process. Here you find only the rants of a hobbyist.