Originally, this case required metal machine screws of a specific size and type to hold the Circuit Playground Express in place. I didn't have immediate access to these specific screws and didn't feel like ordering them online or scavenging through a hardware store, so I designed my own screws in Tinkercad and 3D printed them myself! (Technically they're more like pegs)
First let's make the basic screw shape by dragging in two cylinders. One for the screw body and one for the top. Edit each part accordingly. Don't worry about the size for now, we will adjust that later.
Next, place and group the screw parts together.
Now in a different Tinkercad file, import the case bottom .stl file.
We will use the model to size the screw to and make sure it fits tightly in the holes.
Copy and paste the screw you made into this file. Size the screw so that it fits into any of the holes snugly.
Make sure you leave enough room for the Circuit Playground Express.
Once the screw is sized right and fits in a hole, copy and past three more and you are ready to print!
Save the resized screws into a new file and download the .stl.
And that's it! Download the file and it's ready to be printed!