A koppo is a weapon like a kubotan. It can be used for punching (as a fist load), hammer strike, reverse hammer strike, power slap, pressure point control... etc.
It's very simple to make. Get a 1/2" steel rod and cut it so that it extends 1 1/4" past both sides of you hand. Next get a 1/8" drill bit and drill 2 holes in the rod. To determine where the holes should go, hold the rod with both ends sticking out of your hand. Now, make 2 marks on the steel; one on the outside of your middle finger, one on the outside of your ring finger. Drill on the marks. Once that's done, use a 1/4" bit and drill halfway through, drilling along the first hole. (Please forgive my lack of pictures for the process, but it's simple enough that words will suffice)
Get about 6" of gutted paracord, tie a simple knot in one end, and thread it though the countersunk side of the hole. Now thread it through the other hole on the non-countersunk side. Look at the pictures for help. Finally, adjust the paracord so that it fits snugly over the middle and ring fingers of your dominant hand, remove your fingers from the loop, tighten it slightly, and tie another simple knot. Pull the knot into the countersunk hole and you're done.
How do you use it? It's simple. Punch, hit with the ends, slap, jab into the solar plexus. The beauty of the koppo is that it's hard, if not impossible, to drop. I'll give you some links if you want to learn more
An article on the koppo
Continuation of the first article
Another article, from a different source
Pics 7, 8, and 9 courtesy of Phil Elmore.