Over 20 years ago, a friend of mine had a taxidermy supply business and was considering marketing his own brand of DP. he had all the brands on the market analyzed for comparison to what he was formulating. I saw the report, although I don't recall the exact numbers, they were pretty close to what I'm going to list.
Red, white, black pepper
Misc small amounts of other good stuff
Make your own? Why not! It appeared that a combination of all, some, or most of the above will do the trick nicely. Or just use Borax.