This is a beefed-up, reinforced version of the hold down brackets used on Hunter Douglas horizontal blinds. I found that the part already offered by another user on this site is an exact copy of the OEM hold down bracket. Those brackets are made from injection moulded plastic, and would be too flimsy as a 3D print. How do I know? I printed one and it broke after 6 months.

I modelled this design from scratch and made its geometry thicker. This reinforcement should help keep it from snapping.