My parents' house has a circa-1980's water manifold in the basement, and it came with a small plastic key that's used to turn off individual outlets. That key broke, and the newer ones have a different shape (diamond vs D-shape). This is a replacement for that key.

I printed it at standard quality, but with thicker walls and 100% infill and it seemed sturdy enough. It did need to be filed down just a little bit on the inside in order to fit over the D-shaped knob, but there was enough clearance on the outside to fit between the neighboring knobs.