Last year I finally upgraded from the traditional style bike speedometer (with wires and magnets) I had been using for over a decade to a GPS-based computer, the Garmin Edge 25. Compared to the larger and more expensive ones, it's pretty basic, but it does the things I need, and it also supports external sensors. Over the winter I acquired my first external sensor: the Garmin Varia rearview radar. It allows a rider to monitor the location of vehicles approaching from behind within about 300 feet, and also functions as a tail light that gets brighter and blinks faster as cars approach. I really like it, and I think it's especially useful here in west Texas, where there are a lot of narrow roads with high speed limits, and enough wind to make it sometimes difficult to hear vehicles approaching from behind.
The Varia uses a Garmin quarter-turn mount, and comes with a seatpost version of the mount. Two of my bikes have Topeak rear luggage racks that seem to me better mount points for the Varia than the seatpost. The Topea racks have a bracket on the rear for attaching a reflector (included with the rack) that looks like this:
K-Edge makes a number of different after-market mounts, all of which use the same plastic insert. I could have tried to replicate this myself, but as I've discovered from buying a less expensive after-market mount, a secure attachment requires getting the dimensions of the interface points right, and K-Edge have clearly already done so, so for $5 I can leverage their design and just focus on the interface with the Topeak rack. As shown in the picture, the insert comes with 2 M3 angle-head screws which are designed to screw into the metal mount. In this case, I needed a bracket that was thick enough to accommodate the screws and hold corresponding M3 nuts on the back side.
The design I came up with is this T-shaped bracket, which I had 3D printed by Shapeways.
It is 4 mm thick. The angled rack attachment holes are for angle-head M5 screws. To attach this to the rack requires 3 angle-head M5 screws and corresponding nuts. I used 12 mm screws and nylon-insert lock nuts, but plain nuts will probably work fine. Two M3 nuts are needed because they are not included with the K-Edge mount insert.
Bracket installation begins with inserting the M3 nuts into their corresponding recessed holes on the back of the bracket.
The bracket is then attached to the rack with the 3 bolts.
The K-Edge insert is attached to the bracket using the M3 screws. This is done after the bracket is already attached to the rack because the K-Edge insert slightly overlaps the top M5 screw.
At this point, the Varia can be attached.