Ran across this site with downloadable PDF designs for building outdoor - and indoor - tomato earthboxes out of 18-gallon totes.
The do-it-yourself instructions give a $20-odd price tag to something that requires like $300 worth of tools to build correctly. (And the cage superstructure costs quite a bit for materials.) But he does have interesting pointers on using an EarthBox / GrowBox / his own EarthTainer.
One thing he recommends is laying landscape fabric at the bottom of the box, to keep roots out of the water reservoir. Aside from making cleanup difficult, he says the tomato flavor is hydroponic-vapid if you let the roots reach the water.
I gather he's also investigated an assortment of potting mixes, and claims remier Pro-Mix BX is best, but Miracle Gro normal (*not* moisture control, as we've already noticed ) is good enough if price is important. But he mixes it 5 scoops of Miracle Gro mix to 2 scoops of Perlite.
He also has a design for a floating water level indicator for the fill tube. I dunno if Earthboxes come with an indicator, but that was one of the questions that led me to prefer GrowBoxes - I can see the water level. I also have a high-rain environment - want to maximize use of rainfall, rather than minimize evaporation. But all these systems work great...