I also wanna know the same thing.
Could it possibly be a roll-up Yoga/Camping mat?
Interesting question, I'm gonna join too.
K so here are the labels on the stuff I've been using. To my knowledge, the medium thickness material in black is 5mm, and the thinner stuff is 2mm.
Looking amazing!! Love what you've done so far cant wait to see more.
Looks like some great work so far my friend
Thanks fellas!