It is common for rear blinky lights to come with a seatpost bracket for mounting, however a lot times the seatpost is blocked by baskets, cargo, and other methods of hauling. For example, this basket is on the back of my bike and it makes the seatpost useless for mounting a rear light.
Reader John, who also submitted a hack for turning a .50 cal ammo can into a cooler, was not satisfied with the seatpost bracket and came up an alterative way to mount his light. Take it away John . . .
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I'm a bit tired of how rear lights all have seat post mounts or non adjustable rack mounts. I took a small stub of 1" copper pipe, drilled one hole for the screw and a larger hole for an allen wrench or screw driver. I wrapped the pipe in reflective tape for extra visibility and I thought it just looked better than copper.
Then just attach the seat post mount to the pipe and you have an adjustable rack mounted light.