Traditional 3D printed materials aren't ideal for high-stress environments like drone racing or acro freestyle proximity flight. They're just not strong enough to take the stresses involved. A new material, thermo-plastic polyurethane (TPU) seems perfect for many mini quad applications. It's tough, but also flexible enough to take a hit and recover.
Wontee's Xiaomi Yi protector can be downloaded from the Thingiverse link here: