M3 spanner. I used sketchup to create this and you can download the file if you want to scale it up to other sizes (thus I didn't write the size into the part). I use it for getting under the heatbed to tighten one or two bolts. My printer has a magnetic heatbed so having a plastic spanner made it easier without scratching the underside. It was printed on my Prusa MK3 and took about 11 minutes at .2 layer height, 20% infill. It is 90mm long and 3.5mm high, simple but strong. It fits the M3 bolt perfectly on both ends.