My approach would be to combine pretty much what everyone else has said, and add annealing to the process:

1. More walls, higher temp, bricklayers (the usual wank when you’re more into 3d printing tech than design). Test if watertight and proceed to #2 when it obviously isn’t
2. Can I just buy the container?
3. No? The annealing could be a solution. Take the print from #1 and put it in a baking tray. Fill out every bit of air with sand to support the print. Bake the print at just above the glass temp for the filament for a enough time for the sand to get up to temp. Turn off oven and let the entire thing cool down slowly. Test the dimensions and watertightness.
4. Realize that I didn’t do a good job of supporting the structure and I spent too much material printing extra walls. Others have suggested coating the print, which will add material in a somewhat unpredictable manner. Is it acceptable in view of my tolerances, can I alter the design to make it acceptable? Print with minimal material and coat with epoxy.
5. Get tired of the project and start comfort printing cute stuff for people you like…