drill 2 holes in the lid (slightly larger than you chiller OD) then use a grinder/dremel/tin snips (depending on how thick the lid is) to cut slots parralelle to each other out towards the edge of the lid.
I bought an aluminum stock pot lid that fit from a restaurant supply house for $17 shipped. Cut like butter with a hacksaw. I couldn't bring myself to cut the SS lid. As it turned out it was one of my few good decisions as I bought a new boil kettle and my old one is now my mash tun and wouldn't work so well with a notched lid.