Any ideas on locking tubs from coming out of rack?

I was wondering if anyone has any sort of stopping/locking device on their racks do the tubs don't come out unless you want them too?

I have little plastic stopper that rich from RBI gave me(come with all his racks) and that's why I love his rack, easy to check make sure all cages are locked by checking the little plastic pegs. Could drill holes in the sides of your rack and put wooden fowls across(did this on my old homemade rack)

My bf put in poles to keep the tubs from sliding out - works great. Here's a pic of what I'm talking about (this isn't my photo nor not my rack, found the image on google as I don't currently have pics of our racks):

http://s428.photobucket.com/albums/q...sort=ascending

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

Any idea where I can purchase the plastic stoppers?