Some refillable K-cups come with a center hole so large water often leaks out around the top when brewing. In fact, I can no longer find any with a smaller hole. 
This not only is messy, it also prevents some water from getting into your brew. I designed this very simple adapter to reduce the hole to a size that doesn't leak.

I print them in PETg. Some people will argue that it's not safe to use these for food (coffee, tea, etc.). I have been using them for 2 years without issue that I can tell. And, I found an article from a doctor who studied microbiology and did testing on the safety of using FDM printed objects with food. He concluded that simple, ordinary cleaning will bring FDM printed parts up to operating room standards. I recommend reading it, then make your own judgement: 
https://lt728843.wixsite.com/maskrelief/post/the-final-say-in-food-safe-3d-printing