It looks like you might have room in there for a milk-crate - the heavy plastic kind they sell for storage. If you turn it on its' side and secure it with the open top against the front of the nest box, your little mama would be safe from the others and you would have room to put food and water within her reach. The grid should be small enough to keep the peeps in and safe, as well - and all stay warm and protected.
Good Luck!
Good Luck!