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Don't know the actual diameter of either - dampers are a standard size as far as I know. For tubing you can use this: https://diyearphone.com/products/acoustic-tubing
Looking at the Knowles site, they have "plugs" and "screens" which are different sizes. You want the plugs, which are all ~2mm diameter.
Those new (ish?) Knowles multidriver packages are nice, and cheaper than buying the drivers and making a crossover. I used the GQ-30783-000 2 ways in a Shure SE shell re-fit that sound fantastic. I have the GK-31732-000 on my bench waiting for a home.
I tried using a rotator to make a hollow shell with a two part acrylic, but couldn't get good results. Yeah, because you need to tilt it back a little to keep the material from pouring out you get thicker walls in the canal and thinner walls in the body. Another option to think about... I recently made a pair of earphones using Shure SE shells (linked to an ebay listing below). They actually came out really well (unfortunately I didn't document the process otherwise I would have made a new Instructable). You don't get the same isolation as a true IEM, but the process to make them is significantly faster and easier.http://www.ebay.com/itm/C0-Repair-DIY-Unit-Housing-Shell-for-SE535-se425-se315-se215-W40-Earphone-/131499238359
I'm not saying you can't, just that you'd be better off without.