You could put coffee filters over the holes, then fill with dirt. This should keep most of the soil in your box when you drain it.

Good luck with your box

Here is a link that might be useful: How my garden grows!

Excellent idea. Thanks!

I use my old dryer sheets to cover drainage holes! I was afraid coffee filters would decompose... do they? I don't know what the dryer sheets are made of, but they seem more durable. And work great!

Anything you use will allow nutrient rich water to drain. Dryer sheets may not decompose, but a tight galvinized mesh would work better.

Maybe a more complex system of recirculating water similar to Patti's fish tank concept. I'm not the best one to come up with something, but it's an idea.

Of course, instead of a single box, you could utilize multiple Self Watering Containers. See, they're self contained as well, so no leakes and can be placed on dollies so they're portable. An 18 gallon SWC weighs about 60 pounds full and you can plant 2 tomatoes in it.

Good luck and let us know your progress.

Here is a link that might be useful: Sinfonian's garden adventure