Can can drill a little piece of wood to each shelf of the rack to swivel downward. When the piece is turned to the side, you can slide the tub out. When the piece is turned down, it locks the tub and prevents it from coming out.

http://cdn2-b.examiner.com/sites/def...7733913242.jpg

I know this is a photo of a rat rack, but same concept applies.
http://www.rodentracks.com/images/Rat_Rack_4stack2.JPG