I have a few questions about c&c guinea pig cages?

I have a few questions about the guinea pig c&c cages. 1. Can you use them for rabbits too? 2. I know that i need to put a top on them because i have cats but, how will i make a door to get in and out of the cage. 3. I am planing to do a 2 grid by 4 grid cage but i want to know where i can get a sheet of the coroplast (corrugated plastic) kind of cheap. 4. how can i make a hay hanger out of the grids? Thank you and if you can find out all of the questions for me than that would be a big help! :)