I had an old wireless router power supply (12 V, 1.5 A) that was unused and thought it would be nice to have a variable output voltage power supply. I achieved this by using a cheap DC-DC buck converter and a 20 K ohms potentiometer. The DC-DC converter originally came with a small 20 K ohms multiturn trimpot that needed to be adjusted using a screwdriver, very unpractical. I soldered a potentiometer in its place and put it on top of an enclosure with a small knob to adjust the output voltage.
I designed the enclosure in Fusion 360 and 3D printed it using blue ABS filament. With a pair of M3 screws and stand-offs, I was able to firmly attach the components and create a pretty solid device. I soldered a 4-pin fan connector at the output.
The voltage input is 12 V and the output may be adjusted from 0 to 11.2 V. The 0.8 V drop is not a concern for my application, since I use it to run PC fans at a low speed, usually at 6~8V, in order to reduce noise.