On his blog Ross Enamait pointed to an diy reaction trainer.
Edit: Ross added a quick explanation on how to make your own.
You need:
- hat
- tennis ball
- elastic cord
is one fighter who often trained with a similar device. You can see an example at the link below:
Here are some pros using it.
Vasyl Lomachenko (at 3:34)
Kostya Tszyu (at 0:10)