Bottle cap crocheting

I found some interesting instructions and patterns for making bottle cap trivets with beer bottle caps and #10 crochet thread. They're vintage patterns, invented when most bottles used the same kind of cap, nowadays, you can only get them on beer bottles, or order them online apparently-something I'm not willing to do, nor am I going to go to a bar and ask for their used bottle caps. However, there are always ways around most things and if I want to try this, I'll find a way. I've already got an idea or two that might work, all I really need are similar-sized inserts that won't melt or burn if something hot is placed on it.