Seal it with cement - then line it with rail road ties. Probably 4 sections thick. Have the ties cut into cubes, then you could replace the cubes as they get shot up.
"But I don't want to go among mad people," Alice remarked.
"Oh, you can't help that," said the Cat: "we're all mad here. I'm mad. You're mad."
"How do you know I'm mad?" said Alice.
"You must be," said the Cat, "or you wouldn't have come here."
RAGE AGAINST THE MACHINE
The Critic formerly known as Kodiac