Test tube plug
thingiverse
I've been using this custom plug for 15/16mm test tubes to handle my dry material testing needs, and so far, it's performed flawlessly - however, I'm pretty sure it wouldn't seal properly if water were involved.
Download Model from thingiverse
With this file you will be able to print Test tube plug with your 3D printer. Click on the button and save the file on your computer to work, edit or customize your design. You can also find more 3D designs for printers on Test tube plug.