This is a cap for a precision knife.
I made this cap because there was no cap when I bought it because it was said to be useful for peeling off the support.
It is designed with a diameter of 6mm, but I think it can be scaled down and used for other purposes. The inside is tapered so it fits comfortably.
 

https://www.amazon.co.jp/gp/product/B00J7BNJKG/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1