Please watch this video for more details and to see how I did this: https://youtu.be/FfepDrXhgJA
This is a 3D printed chalkboard or whiteboard (depending on how you paint it). I just created a little plastic base, sanded it, painted one with chalkboard paint and the other with dry erase paint, and then attached the borders using some glue. I printed the borders out of Colorfabb Woodfill, but you could use any material you want. Then you can draw away!
There's also a speech bubble that you can make into a whiteboard or a chalkboard. The backs are flat as well, so attaching a magnet would be really easy. That way you can put it on your refrigerator.
Here is the list of materials you'll need to do this:
Sandpaper (100-400 grit)
Printer Brand: Robo 3D
Printer: R1 ABS + PLA Model
These are really pretty simple prints and there's not any details on the sides, so layer height doesn't matter too much. Infill isn't that important either, because you probably won't need your chalkboard to be super strong.