Just needed a T-fitting for my car and was in a hurry, I designed this T-Shaped fitting.
To make it more waterproof, after printing, you can "PAINT" it with some liquid epoxy or bi-components glue, something to seal any possible nanofisure.
Size is for 6mm inner tubes, but if you redimensionate it , sure you can adapt to your needs.
You can print in any material, but I would recommend ABS, Nylon, PETg or any strong material.
Mastoner ABS, or Nylon
MAybe you don´t need, but is a good idea to seal it just "painting", with a liquid epoxy or resin, or something that seals any possible nano fisure.