I'm building home made versions of the earthbox, which is a self watering system for plants. I want to grow vegetables for us to eat, plus strawberries, tomatoes, that sort of thing. I'd appreciate some help finding parts.

The way it works is water is sucked up into soil by capillary action from a reservoir in the bottom of the container, but it's separated to avoid getting too much soil in the water. There's a good diagram here, which I'll try to include below.

I have some 90L Sistema containers from The Warehouse, I got them on a two for one sale. I have supports, 100mm PVC pipe. The part I'm having trouble with is the rigid screen, which is what the soil sits on.

The two major materials the DIY people seem to use is:
 - The lid of the container. I've tried this, but the plastic of the containers I've chosen is quite bendy making it difficult to work with, and also cracks quite easily when you cut near the edges. With solid plastic you have to drill lots of holes through it, so air can get into the roots of the plants, and to let water come up. Example image here.
 - Plastic grate, as shown on this page by this image. This example uses ceiling tile grate, which is cheap in the US. I can't find anything similar in NZ though.

Can anyone suggest a rigid waterproof plastic or grate that might work well in there? The box size is approx 650 x 400mm. I want to make quite a few, so it can't be super expensive. Perhaps:
 - Some kind of a grate, that can be cut to fit the box
 - Rigid plastic, but that can be drilled and cut to fit the container
 - I thought about thick wire mesh, but it would rust out eventually, and probably not be great to have in the soil
 - Some other creative idea? I considered wood but it would eventually rot, plus treated wood has awful chemicals in it
 - A couple I found are safety flooring and walkway grating, but I suspect they could be pretty expensive

Any bright ideas anyone?