What are the dimensions of your cage (how many grids x grids)? Do you have any pics? If so, I can give you better advice.
You can make a door out of two or four grids, depending on the size of your cage and zip-tie it to one side of the door opening. Try and make the door opening on the first floor so the rabbit can hop out/in easily. The second floor would work okay if you built a ramp. Use lanyard clasps or quick links to close the door to the opposite side of the cage.