A Kuboton is a short, usually around 4-6 inches, metal rod used against bones and nerve points. It can be very effective in hand to hand with a little practice. I have carried one on my keychain for 20 years or so.
Technically as 67390FE said it is a wooden (originally) or metal rod...however in training and street application anything you can get into your hands like a cell phone, handgun magazine, unopened pocket knife, cordless telephone will work.
If you have the competency and confidence to use one (through training & practice) then anything you can get into your hands will suffice and you don't need to invest in a "specialized" piece of gear...if you have the luxuary of using a dedicated "hardened" Kubaton with a key ring full of keys then it will be a little effective but probably negligable.
When I crawl into our FIST suit and let folks pound on me with harden Kubatons or any improvised tools I can't tell the difference.
The focus should be on develop one's software and not get wrapped up on the hardware...once the software is wired correctly then you start seeing yourself as the warrior/weapon and the world is one big Wal Mart full of improvised tools at your disposal.
Your assessment is spot on. We teach the same techniques for the kuboton using common items, ink pens, pocket flashlights, etc., in the dojo. Most anything can be used as a weapon in the right hands. It all goes back to training.
Just remember, don't bring a knife (or a kuboton) to a gun fight. Train all levels of defense, so that you can respond with the appropriate level of force.
The best thing about having one on my keychain is it's harder for me to lose my keys