A long time friend of ours raised meat rabbits and made a 100 x 20 foot pen with big PVC pipi as the underground runs for the younger rabbits to get in out of the sun/heat. The also had an underground building with PVC pipes for the rabbit entries but had an above ground door for access by the humans. I know I don't have any photos and they were in their 80s so ill try to draw a diagram and post it.