I thought of a combo-fix, although you might not see the need to fix anything, lol - but if you took Stush's idea, turned it upside down and created a lip , and stapled plastic or painted the shelf, then you could a) stabilize the shelf b) at least semi-waterproof it and c) give yourself even more room by not having to put the drain-saucers under the individual plants.
Regardless, it is a very nice set-up!
