If you ever looked at one of the white Portal sentry turrets and thought it was a blank canvas waiting for a Sharpie, you will love this. ThinkGeek has an 8-inch vinyl DIY sentry turret that comes with an official Aperture Corporation black permanent marker.
That means one mistake and you are SOL. I would be more impressed if the surface of the turret could be wiped clean so you could doodle repeatedly. The turret replica is made from vinyl so perhaps if you had a dry erase marker, you could wipe it clean. That, or you could always hit it with a Thermal Discouragement Beam.
The toy is an officially licensed Portal 2 figure. It sells for $19.99(USD) and is available for your doodling pleasure right now.