I like to keep my mini Pentel Waterbrush in a small watercolor kit I have from a different company. But the brush doesn't fit unless I keep it unscrewed. I'd been using the original waterbrush body that came with the kit since it matches the pentel top pieces, and the plug that came with the original kit, but the body was getting really gross, and now that I have a 3D printer I was able to print a plug that fit the pentel body instead and swap it out!

-- It is printed so that it's a tight fit, so it expect it to take a bit of effort to push in and pull out.