If your water pipe is 12" id and 30" long you'll have 2 cubic foot as John suggest. I have read that the egg crate is not the hot setup. 50% fiber fill sounds better to me.

If this driver is not suited to mount vertical, You might find a nice spot for it to lay down along a wall or behind the couch.

If your driver is not going to fit exactly into the tube. You can cut a ring from 3/4 ply. Predrill and screw the ring to the tube radially and you can then mount the driver to the ring which has now become a face flange to the pipe.

I like Wayne's Idea. lots of options for the parts you have.

Bill