I know you asked for a homemade recipe. You've gotten some good replies and I'm sure you could find many online. I just wanted to chime in with my recommendation for the best store bought brand. I worked at a retail fish market for 7 years and I have tried all kinds. Make sure to serve your sauce (homemade or store bought) cold.
IMO, the best store bought one is Crosse and Blackwell with a green label. I recently had an opportunity to taste other brands and it reconfirmed for me that C & B is by far the best.
http://www.crosseandblackwell.com/products/Products.aspx
The fish market I worked at also made their own. The base was chili sauce doctored with horseradish and a few other things I can't recall.