The reflectors can be readily attach to the spokes - even with flat spokes. This probably helps the cross-shaped inner structure of the spokes sticks. Despite some loose seat even after the first mountain biking even all reflectors on the wheel.
Tip for attachment to flat spokes: The reflectors loose plugging. Then hold only the spokes approach and turn it 90 °, possibly even push some towards rim. Because the spokes approach is around can be rotated so good the reflector.