i assume u mean the wierd thing (which is like a blanked off cold air feed hole) which is in the oppersite corner to the air feed hole?
im not sure of the size phil, id gues its about a 3" diameter.
im surprised ur lower box is different to mine... i assume that pic is of ur lower box correct?!
when i first saw this odd thingy, i thought of the possibility of having an extra cold air feed, but to be honest i dont think its worth the hassel for a very minimal gain that you may or maynot get! also surely you woiuld lose some pressure in the box by doing this... which i read was bad thing!

plus where could you get the cold air from?