Well you should not leave your guinea pigs unsupervised or unwatched to allow a snake to crawl in. However if you did want an easy way to make sure no unnoticed small critters got into the cage just get a roll of screen door mesh (I recommend the aluminum over the black carbonate) and use very small zip ties or wire to attach it on the outside of the grids. I once tried it to keep bedding from flying out
http://img.photobucket.com/albums/v2...s/P1010047.jpg. It works but wasn't worth the effort for very large cages that I kept redesigning.