Below is the Crazyflie promotional video. Jump to 2:30 to the part that shows their "training wheels".
Step 1: Buy Crazyflie
Step 2: Step 2: Make Loops
I used a 50mm piece of PVC to wind the wire around to achieve the desired loop size. The wire was twisted off. and one of the tails was cut short and one long.
Step 3: Make the Mounting Point
Step 4: Tie Loops Together
Step 5: Welding for Final Position
Step 6: Fly and Crash Gently!
It does however, bounce nicely off the floor.
The result wasn't exactly as I hoped. Plan is to rebuild with the bumper higher (in line with the propellers).
The tags underneath that I threaded through the vias also helped to make a springy landing gear which is an unplanned feature that could be expanded on.