I tried using 1mm (I’m using my kitchen oven as a heat source) but I couldn’t get it pliable enough and wound up with 10 failed forms so I ended up getting some .5mm on amazon Set my oven to 350 and put my frame in, I found the best thing to do if you’re using an oven is to put the plastic in as the oven is getting up to temp rather than when it’s already at temp. I guess the gradual increase and consistent heat helps it get nice and soft. Plus the thicker plastic was turning white as it started to sag, could be I was doing something wrong since it’s my first time though