Is there a way to make a watertight model? Is there a spray that may work?
There are several ways to ensure water tightness:
Thicker walls
I have experienced that 3 walls and 5 tops/bottoms with an outer shell of Concentric while the core is lines become watertight enough that leakage is almost none.
Lacquer
The oldest known method to seal a piece of porous material is a lacquer, followed by glaze. Glazes require to be burned at some thousand degrees, so are out, but lacquers are simple and easy. Basic clear acrylic lacquer can be used to seep into all the cracks and crannies and then harden out. It's easy to apply and cheap from home depot. Polyurethane lacquers also work. If you look to use it for food, ask if the lacquer is food rated in home depot.