Print one K9 adapter and four strap holders.
Attach the strap holders to the adapter using #4 X 3/8" countersink wood screws.
Thread 1" wide Velcro strap through the strap holders. You need about 12".
You also need four 1"x1", 1/8" thick self-adhesive rubber or foam pads for better grip, and not mar your flash. Adhere them to the inside of each strap holder.
The adapter is design for the Canon 580EX and the Yongnuo YN568EX. It will probably fit other flashes by adjusting the thickness of the rubber pads.