A very simple design for a classic key. I printed a few of these for a cabinet whose original keys were missing. They work well, although if too much force is applied (i.e due to rusty or not smooth-turning lock) it may break.

Note: this is my first attempt at 3D design ever, and I'm not good at it. So probably there is a lot of room for improvement in this model.

It can be a lot more tricky to print that it seems. I had plenty of trouble printing it in the original position, but eventually it worked. Use a raft or a brim to improve the adherence to the bed and to provide a more stable foundation to print the handle, or print it upside down, but then support will be required.

For the best strength, use 100% infill, and make sure that the gap surrounding the hole at the bottom is correctly infilled by your slicer (you might need to adjust the extrusion width for this).

Finally, if you have heat issues, print two or three keys at the same time, so that each layer has enough time to cool down properly.