I’m going to deviate from the norm on this one. This is a project I worked on years ago as an Archaeologist, and it remains the design achievement of which I am most proud: engineering and fabricating a flotation machine for soil samples.
It may sound like an odd example of design, but when I began working in an Archaeology lab in Tucson, I was non-plussed to see that soil samples were processed by running the water through the sink in a precarious setup that involved carefully balanced buckets. Not only was it a terrible waste of water in a desert landscape, but it was hokey and prone to spills. So I took it upon myself to come up with a recycling, standalone system that could be constructed from inexpensive materials I got at the hardware store. The new flotation devices were portable, used far less water, and allowed any spills to be mitigated by recollecting the samples from the water reservoir.
The company liked the machines so much that they paid me to make several more and hired me out to build a device for another firm in town, using the design and devices for years to come (over a decade now).