In addition to plugging up all the holes, you
should make sure that the surfaces
surrounding the noise source are soft (not
hard and reflective) and bumpy. For
instance if you have a cement block wall
that is unpainted, it will be better than one
that is painted because the paint fills in all
the holes and reflects the noise back into
the room.

I've heard of fero cement hulls,
but concrete block?????

What sort of mortar do you use?
What shape hull can you build?

;-)