I supported the CoffeeJack project on Kickstarter. I got my CoffeeJack, but unfortunately my tamp has not yet arrived. This will work in the meantime.
As this is a object that comes into contact with food, I would recommend:
1) Use a Food Safe Certified Filament
2) Seal the tamp with some sort of glue or sealant before its first use.

I printed at 20% infill, seems strong enough.

This object was made in Tinkercad. Edit it online https://www.tinkercad.com/things/fdmIFAca1ic