Small magnetic sensor for bike computers. It is easy and fast to print, 3g filament about 30 minutes. The 3mf contains 4 different (1.6 - 2.0 mm) mounting blocks that cover most common spoke diameters. Choose the correct diameter by pressing the two halves of the block firmly against the spoke. There must be a small gap between the two parts. 

Two magnets 6x2 mm or one 6x4 mm are required. The ones I have here can be pressed in by hand and don't need gluing. The magnetic dimensions may vary. 

Make sure the straight block is facing the spoke mounting slot on the back. Adjust the position before squeezing everything tightly. You may not be able to disassemble the unit without destroying it. However, this can be easily done with a side separator or other pliers.