you don't need a lid or foil with a hole on the coffee can. leaving it wide open works fine. As long as the flames don't wrap up and around the top of the can, the cotton shouldn't flare up. if it does, take it off the fire and cover it to suffocate it.

I use my barbecue grill and just set the can on the grill and keep an eye on it.

I made 4 pounds worth the other day this way.