This is something I made for my engineering class in AutoCAD a little while ago as a mini-project. 

Here are the steps I took to make this:

1. I created a 30x30 unit rectangle and put it in the DEFPOINTS Layer
2. I used the MTEXT command to create that one letter and scaled it to fit in the box
3. I then used TEXTEXP to turn the text into lines and trimmed the rogue ones
4. I added a little keyring (forgot the diameter) to place on a keychain
5. After that, I used the EXTRUDE command to make it a 3D solid
6. Once it was all finished, I used the STLOUT command to save the 3D model as an STL file

So yeah, just some basic modeling on AutoCAD, nothing too special… Hope you enjoy!