I chose this method after discovering that threadless handlebar stems don't expand to 31.6 mm, the diameter of my seatpost. Stoker bar stems aren't manufactured to this width, either. The only other choice was to buy a seat post shim, a narrower seat post to fit into the shim, and a stoker bar stem - at least $100 in parts. Thankfully, I had the rack mount on hand.
Since taking the photo below, I moved them up the seat post for easier grasping. My son feels very safe with this rig.
-Phil
