I made this mount to attach my bike light to my bike stem, and free up space on the handlebars.
The light clips into the round end of the long piece which is bolted to the u shaped piece which is "rubber banded" to the stem. So Additionally to these parts you need an m5 bolt, nut and an o-ring/rubber band.
The light is a pretty standard style that normally fits into a flexible clasp. Google pedal craft light or any number of others. I have like 3 from 3 different brands.