One of the issues with only have a thin panel is how to hold them together. If they’re only 3mm thick and you can’t have a nut on one side you don’t have much material to grip the screw. One solution, or aid, is to first punch the hole through instead of making a clean hole. This leaves some metal on one side and gives more thread area to hold everything together. Check out the example:

Screenshot 2014-07-29 22.41.47