You could set the entire cage over two plastic storage boxes, with the cage divider centered over the walls of the boxes. Each cage side would drain into a separate box. Look for dog crate trays, bird cage trays, hydroponics flood table trays, cement mixing trays, appliance drain trays, all sorts of ideas. You could build your own trays from cut to fit rigid plastic glued together with silicone. Coroplast can work for cage flooring depending on how heavy your plant pots are (but you could set the pots on braces).