One way to do it is to just attach grids with zipties across the top.
If you have cats and dogs that you are concerned might actually get on the top and attempt to walk around, you might be better off going to Home Depot (or whatever home improvement type store you may have near you_ and purchase the metal closet shelves to attach as a lid.
see these two gallery areas:
http://www.guineapigcages.com/photos...ry.php/cat/508 http://www.guineapigcages.com/photos...ry.php/cat/533