The "El shield" was indeed what I had had in mind -- As Kathy pointed out, everyone else (well, except for the Kents and, eventually, Lois) thinks it is an "S" for "Superman." It is also physically near Clark's heart and he literally stands behind it.
The suit would work except for the "standing behind" part. But if that part were taken figuratively, then it could also solve the riddle.
Thanks for the feedback. :-)
Joy,
Lynn