Subject: Thickness of copper and largest holes allowed when blocking 13kHZ RF signal

I am trying to block RF signals being made by a 12V DC motor on a kid's ride-
on toy. The signals are being picked up by a hobby type metal detector that 
runs in the 13kHZ range. I read the response on wrapping the motor in copper 
roof flashing but can I make holes in it to allow the heat to escape or would 
copper screen (like house screen) be enough to block them?

Re: Thickness of copper and largest holes allowed when blocking 13kHZ RF signal