I absolutely love this idea! Probably totally obvious to many but not to me! I’m gonna try this.
Thomas’s instructions for making it out of an old bucket, newspaper and some seeds are here, and he links to more instructions at seedsofknowledge.com.
I did make the upside down thingy on my own. Took a standard 5-gallon bucket and drilled a 2.5 inch (55 mm?) hole in the center of the bottom of the bucket. Lined the bottom of the bucket with newspaper (you can use a coffee filter, too) and cut an X in the bottom where it aligned with the hole in the bucket.
Suspended the bucket between two chairs so as to be able to get the plant in there, then fed the tomato seedling through the bottom so the roots were in the interior of the bucket. Packed some peat moss around the roots and then topped it off with potting soil within an inch of the top of the bucket.
Water liberally and away it grows! You can’t see it in the pic, but I’ve planted parsley in the top of the bucket (you can plant basil, too) to complement the tomato. They grow well together.