we did this for some folks at a care center, the bottoms of the boxes were filled with creek stones, for drainage, we added a solid aluminum bottom angled so the drain hose drained down the outside pillar, back to a reclamation tank, thus recycling the water for all the gardeners to use.

We also installed standard raised wheel chair accessible beds.
the link below is a .pdf file, with the material list, plans and pictures for those
Building Wheelchair Accessible Raised Garden Beds