haha.. The method I use would be better if it were situated on a bench, but I am working with limited space and budget as well. I took a sheet of plywood that is slightly larger than the footprint of all three tanks and then nailed a 1" x 2" border around the perimeter with the border being 2" tall. I then took TWO layers of extra thick black sheet plastic ( just in case first layer gets a small tear) and fit this into my "Basin" making sure that when filled with water the plastic would not be pulled away from the walls ( leave a little extra slack around all edges) I then tacked in plastic with finishing nails. Note: You don't want to put any nails in any area of the actual basin or you compromised waterproofing.. Only nail on top edge of border trim. Note #2.. you need to somehow put tank on stilts to keep out of wastewater. I used small sections of PVC I cut and set tank onto 4 pieces. One in each corner, but you will need a second person to assist in placement of cages ; Drill a small drainage hole in the middle of each tank if not already in place. ( The water can build up with my misters running ~ 10 gallons a week with only needing to be drained ~ once a week. That said, eventually when I have house and ideal setup, I will put this basin on top of a table with a drain valve manufactured to fit in the bottom and this simplify. I currently have improvised and purchased a python brand water siphoning system and drain into nearby sink.. just my two cents, but thought I'd give you an idea of my concoction 
