With the addition of a toorie (that ball/pom-pom on top) and a cockade and badge to the left temple, even a lowly cheap-store beret can become a passable balmoral. Add a coordinating headband (1.5-1.75 inches) and it becomes even closer.

I'd second the recommendation to Thompson's book. I intentional refused to get it for years, but I regreted that decision after I got it. The appendices are well worth the book. I use them ALL THE TIME!!!!

Now, I make the toories and cockades for all my own balmorals/tams. I can do them in normal black or red, navy, or livery colors.

It also has a section on conversion of a jacket.