K-Ts right, the SC3 is the most compact, yet still tough. Also, the MN0x is a small lamp, so the clever holder is small, and you don't waste much space if you don't use it. On the flip side, you definitely cannot get another 123 in there, even if you got the holder out, which is molded into the body. The space is only about the diameter of a pencil.
Now, if you took the lamp holder out of the SC1, there would be space for a seventh 123, but the molded lamp holder also keeps the 123s from rolling about, so your new problems would be 1) if not full, the batts might not stay in place and 2) the middle of the carrier is taller to accept the lamp, and your 7th batt would rattle, if you didn't line it with foam or something. Finally, the plastic is pretty hard, and something might crack if you try to drill out the lamp assembly holder.
cheers
daloosh