I used 3mm. It took a couple days. I still want to plasti dip spray it. I dont like the shine the mod podge did to it. I used the arkham origins foam pep file. if you use that you might have to scale it down.
Update: I have a couple more layers of plastic dip sessions and it will be done. It's looking better already. I don't like the gloss mod podge look, so now it will look more stealthy. I'm also thinking about cutting out ear sections and putting a fabric in place so I can hear better. Any suggestions?