You could try using a soldering iron to melt around the edge of the hole, and then file it down with wet-and-dry or sandpaper. This worked when we were making a plastic bar-cover to stop our ham from bar-chewing.
You could also have a look at the two articles about bin cages in the Wiki.
How to Make a Custom Bin Cage
Bin Cages Made Easy!
Hope this helps!
