Decaleur: Velo Orange Steertube Mount
A decaleur is an attachment system and brace for stabilizing your handlebar bag. Velo Orange makes an affordable and simple quick release style stainless decaluer. Available in two sizes, 1" and 1 1/8". These mount on your steertube, where your spacers go.
Note: for maximum stability try drilling a third mounting hole in the center of the bag mount.