I made a little wooden clamp to hold the safety flag on a Catrike. The mount that came with the trike originally was pretty weak, relying on the tightness of a single m5 screw to keep the flag upright.
My dad puts the trike in his car often, so the flag needed to be easily removable. I mortised in a hex head screw which threads into a brass thumb nut to clamp the flag shaft. The little pin above it keeps the clamping plate from flopping around when the flag is removed.