I've added the last two drives for this round of expansion which brings the total for the cluster to 34 HDDs (OSDs).  Amazon would not allow me to purchase more of the STGX2000400 drives, so I went with the STKB2000412 drives instead.  They have roughly the same performance, but cost about $4 more per drive.  The aluminum top portion of the case should provide better thermal performance though.