Building structure

I launched this morning to the site of 2 fisherman gently lowering
cinderblocks out into open water. They must have had about 30 of them on the
jon boat as it looked like it was gonna sink. I fished nearby watching as
the continued to drop these blocks down. I figured they were building some
structure to fish later on some time. When they left I motored over and
marked it on my GPS. I'll see how it does whn I go back out.

Question:

If given the oppurtunity how would you go about adding some structure to a
lake. This particular lake has a very featureless bottom, barely any stumps
or sticks, average depth is between 8-12 feet and it's loaded with weeds.

What materials would you use to construct it, how big would it be and in
what shape? Anything that promotes fish growth and doesn't hurt the lake is
permissable. Idea's? suggestions?