These are some drive bay rails for the Corsair Air 540 computer case (may fit other Corsair models as well).
Also included is an adaptor for mounting 2.5" drives.
To install the 2.5" adaptor, first attach it to the drive using standard 2.5" drive screws, then attach the left (L) rail to the bracket using the larger 3.5" drive screws (they should self-tap into the plastic.
I recommend to print in a plastic that is tolerant to higher temperatures, such as PETG.
The author marked this model as their own original creation.