In the past I've read this: http://amazingribs.com/BBQ_buyers_gu...de.html#partyq
Where he suggests:
On the Weber Kettle, the blower needs to go through a 1" hole that you drill in the grill. Their website and printed materials show it mounted at the bottom, below the charcoal grate, but that is on a Weber Smokey Mountain. I had problems with ash getting into the fan and with it blowing ash around down there. Since then I have drilled several holes in my old trusty 15 year old kettle testing locations (now I have an excuse to buy a new Weber Performer Kettle!), and, if your lid fits tightly, it can go pretty much anywhere between the cooking grate and the charcoal grate
So if I ever would get one (no matter which make) I would fit it between my cooking grate and charcoal grate.