A simple handbrake i made for the Logitech G25 Steering wheel. 

It uses a button on the Gearbox. You can find a tutorial here: 

ASSEBLY:

Bend Switch like this (You may need to adjust for your needs)

Screw Switch in place:

Connect parts:

Slide pin in (you may need to scrape or sand the pin a littlebit so the parts move freely)

Put other pins in (these are for strenght)

Cap the pins with the rings (it is a tight fit, may need some sanding and or glue if you sand too much)

Add elastic band:

Slide the stick on:

Allign bumps with the holes on the gearshift controllers underside: