I've been learning how to use Sketchup to design parts for 3D printing and have now printed my first useful part - a simple mount for an HC-SR04 ultrasonic sensor. There are plenty of designs I could have downloaded from Thingiverse, but I wanted to design my own as a learning exercise. Here's the rendering of the design in Sketchup:
Here is the 3D printed part. This was a medium resolution print in PLA.
The ultrasonic sensor fits quite snugly, but I'd probably need to use some hot glue to secure it in place. This feels like a good first step in 3D design though.