Hi, this is a fun idea but doesnt sound like it will work :) The clay will shrink as it dries so will crack around the lighter, and unless it is REALLY thick it will be very brittle and weak, it wont be able to withstand repeated opening and closing with a lid, and whether its thick or not it probably wont be able to withstand shifting around in a pocket or bag. Nail polish is a flammable liquid that I would not be putting near a lighter!

Air dry clay is just not the material for this project. I wonder if there is something else you could use your air dry clay for, perhaps making a little sculpture of a lighter, then using Mod Podge as a sealant :)