In a pinch I needed a really narrow hex bit extender which would fit a small hole. Designed this to be basically as narrow as possible. When I printed it it cracked when I put the hex bit in, but ultimately held its shape and enabled me to screw the screw I was trying to tighten. So useful in a pinch but probably not a reliable long-term tool.

Measures approx 68mm long, with the extension part approx 58mm. Diameter is 8.4mm.

I recommend printing it tall and upside down (ie the part that goes into the screwdriver or drill facing down) - it'll take longer and you'll need brims but this'll ensure there aren't any supports in the parts that need to fit/a good tolerance.