A mount/holder for the cheap, common Amazon clearance/marker lights. I printed mine in ASA, so they will (should) hold up to UV outside.
These are the lights that fit this model:
https://www.amazon.com/dp/B09KKTJ8J4?ref=ppx_yo2ov_dt_b_product_details&th=1
The author marked this model as their own original creation.