There are patterns on here for larger ones, or ones with a break in the middle for those with larger heads - I haven’t seen one for tiny heads, but I imagine they’re out there too.
Acrylic is pretty standard across the industry - PG comes pre-masked, but if you don’t care about the masking any 1/8"/3mm cast acrylic will do. If you put them in the oven at 180°F (80°C) for a couple hours it’ll “anneal” them and make them less likely to break. If you bring the temp up to 300°F(150°C) you can bend them (with leather gloves or equivalent) to match the curve of a head.
Folks have made them out of thick leather, and the flexible cutting board material too…