My first thought was to use this kind of cardboard to make the container. On one hand I wanted to reuse some leftovers I had here, on the other hand I wanted to create a mould with a specific size, closer to what I imagined. This would not have been possible with a comercially available polypropylene mold nor with a tupperware but using carboard has it's disadvantages as well. It's difficult to make it watertight but I think I solved this by coating the interior with latex.
Superglue was used to glue the parts the best I could.
Another reinforcement: hot glue, which is one of those materials that comes handy every once in a while, just like white glue.
The box looks huge but it actually isn't. We'll see it in the end.
This is important: latex easily ruins brushes! What can we do? Have a small container with warm water and dishwashing soap with a sponge inside to clean it before it gets covered in dried latex.
The thick coat of latex still drying - an hairdryer can be used but I chose it to dry naturally.