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 some wire that my girlfriend uses to make jewelery. the wire is really elastic and not very ductile which means it deforms easily but springs back to its original shape. Perfect for the bumper.
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
Bend one of the tails down inline with the outside of the loop. cut it so its about 10mm long.
Step 4: Tie Loops Together
I used some enamelled copper wire for electronics to connect the loops together.
Step 5: Welding for Final Position
Turn the crazyflie over and use the bottom as a jig. Insert each of the tails of the loops into the vias in the corner of the crazyflie. Using a soldering iron tack the copper joints. As you are working around make sure to lift the loops level. After you have finished welding, pop the loops out and change it to the topside.
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.