Designing anything is often a matter of experimentation.
Over at this post at the Parallax Forums you can find the pdf drawing files and all the discussion about the Niko's Fibonacci Spiral Challenge and a Chalk Feeder Tube for the S2 robot. All of this will work with the new S3 as well and the old original blue.
The final drawings for the S2 Chalk Feeder Tube Project contain three solutions. This is "open source" - of course! ;-) (meaning it is for your use, copying and improvement!) Sheet 2 shows two alternates for the method for providing pressure on the chalk stick. The first - Alternate "B" is my original idea of a rubber band tensioner. The second - Alternate "C" is a spring tensioner. With both these solutions - it is critical that the pressure applied to the chalk be enough for good scribbling, but not too much - if so, the drive wheels of the S2 will raise from the rolling surface. Not to much, not too little, but just right! Think Goldilocks! "Field adjustment" is required. The Goldilock's method is often a key in robotics...
Keep experimenting and keep making mistakes - it is the best way to learn something!