How Do I Keep Baby Rabbits From Leaving The Nest Too Soon?

A few weeks ago we found a domestic rabbit loose in our neighborhood. She was very tame and we easily caught her and called friends who own bunnies who helped us set up a cage for her and try to find her owners. When we hadn’t had any replies to our posted signs and were called out of town for a funeral, we left her in the care of our bunny-owning friends. While we were gone, she had two babies. . We returned into town, but left the three rabbits with our friends for anoter week so as not to stress out the newborns or the young mother. Well, they handled the babies quite a bit (maybe up to an hour a day) and since we brought them home yesterday, they have hopped out of the nest three times. They are only 2 weeks old and don’t even have their eyes open! I am concerned since I read in a bunny care book that bunnies should not leave the nest until at least 3 weeks of age and that if they leave early they can eat food too young and introduce their guts to bacteria too soon, they can get sick and die. My plan is to get a cardboard box with high sides and transfer the nest in hopes of keeping them in longer. Does this sound wise? Any other ideas? Your help is greatly appreciated.