Recommended with reservations
Why Are Ping Pong Balls So Flammable? Lighting 100 Ping Pong Balls On Fire
Why people recommend it
Shared as a warning that ping pong balls burn fiercely, including to someone using them around LEDs in an Arduino project.
Watch for: Keep heat sources away from these diffusers
Some of the recommendations came with caveats. Read the discussion before relying on a detail.