This project is ready to be printed as is. I'd recommend printing the test ring before printing either the Flat or Original version to ensure that your settings / slicing won't have any issue with the little half-spheres that lock the two pieces together. It's also best to use a darker color filament for this object.
I designed this object using tinkercad and the basic shapes provided there. I have made several iterations of this object, and have used similar devices that other designers have been kind enough to share online. I have found this version to be the sturdiest, easiest to clean, and most aesthetically pleasing of its kind. The cap makes a satisfying locking noise and does not wiggle when closed, but is still easy to remove.
Take a single coffee filter and layer 5-7 grams of activated carbon powder (available on eBay, in your local pet store, etc.) squared off in the center. Fold the filter over that square, and apply another coffee filter around it in the same manner. Wrap this filter in a paper towel like a burrito, trying to keep an even layer of paper towel all the way around the filter. This cylinder should fit into the base tube nicely. Insert a wadded-up half sheet of paper towel towards the mouthpiece. Then insert the filter. The half-sheet will act as the first layer of absorption, and will help to preserve the carbon. It will need to be replaced on a regular basis, and the user can replace the other paper towel that guards the coffee filters that contain the carbon as they find it is necessary to do so. When you find that your filters are becoming dirty after having replaced your absorption materials, you can reuse your carbon powder an additional time in another set of wrapped and coffee filters in a paper towel. When you've inserted your half-sheet and wrapped carbon filter in the tube, you should still have a little room inside. Lock the cap onto the tube, and push the materials inside the cylinder away from the mouthpiece and towards the cap using your finger. This will help abate any discomfort / bad tastes from use.
There are very few small crevices to clean, I have been using denatured alcohol to clean mine with no issue for several months. You will notice color fading over time if you go this route. Regular cleaning with alcohol prep pads (such as Curad) is recommended to abate this.