I’ve done something similar in the past, based on this video (in Portuguese). So here are a few tips.

Use some firm material for the box. For example, if using cardboard, reinforce it by gluing multiple layers together.

The black paint is obligatory, it simply doesn’t work well without it. Mine was painted outside and inside. Ideally with a paint that protects the material from drippings.

For the side holes (the video I linked doesn’t have them, but my box had), tulle works well. Mechanical resistance is not an issue, but the size of the mesh is.

For the middle, I didn’t use a net, because it’s a pain to clean. Instead I run a string through the box, like this:

Image

As you knot the loose ends together, the string becomes firm enough to hold a metal grill in place. Then when it’s time to clean you just need to take off the grill, untie the knot, and clean it. Granted I used the box to cure pork and beef, so drippings were a concern, but even if you’re using the box for vegs alone remember some (like tomatoes) are rather goopy.

If you’re using transparent plastic instead of glass, a few needle holes work like a charm to prevent condensation.