Im building a box that will have three parts, does it have to be watertight?

I think it should be a simple design: it has three parts:
The Bottom
The middle shelf (will be painted red)
The lid

The bottom will have a indented lip so that the lid (will use a clear material) can slide over the shelf and contact with the bottom part.
Thus, the design looks like this:



The middle shelf (will slide into the bottom part and be covered by the lid) It is also hollowed out inside to save on printing material. :





A simple lid (will be printed with a translucent material):



When I check for watertight meshes, I of course run into problems... due to the hollowed out design:




Any idea on what I should do to finish this project? Attached are the SketchUp files.

Thanks for reading / any advice.

Apart from that everything is OK.
Get a copy of http://www.netfabb.com/basic.php
It will check, repair, measure and scale. It will also show the volume.
On this model. Top - 141.31 cm3.
Middle - 319.33 cm3. One wall is showing up as 6mm thick.
Bottom - 181.09 cm 3
Box complete - 645.73 cm3

Gratis

Thank you so much for your time! A real learning experience!