If you happen to own or have access to a 3D printer, the following addons might be of interest to you.
Bitcraze Crazyflie frame by VGer: A protective frame that attaches to the motor mounts of your Crazyflie and includes some safety loops to protect the propellers. Also allows to mount the battery beneath the Crazyflie thus lowering the center of gravity. Weighs around 3 grams.