As I wrote in the Beef Curry recipe, I don't make my own curry powder. Lomo asked in the comments about the "secret" 15 to 20 spices that make up curry powder. After poking around a bit on Japanese web sites, I came up with this page that describes what goes into S & B curry powders, the most popular brand by far in Japan. It's an official S & B page, so should be accurate, though as you can see the percentage given have a pretty wide range. I guess it's because the actual formulas are 'secret'. In any case it gives a starting point for any experimentation I think.
I've also included a recipe for making garam masala. Note that I make no claims whatsoever that these are authentic mixes for Indian or other curries, but I'm talking here about Japanese curry.