On Tuesday, September 1, 2015 at 4:39:04 PM UTC-5, Don Y wrote:
Hi,

We have a "double wide" (17'?) *insulated* garage door. As such, the
internal structure of the door is largely hidden.

Short of putting carriage bolts *through* the door (the exterior is
"smooth"), what other approaches can I take to secure the stiffeners,
hinges and other hardware to the door panels?

Mine is an insulated Masonite Phoenix brand...the 2 lower panels have the stiffeners you describe. They are attached to the lower carriage bolt of the hinges. So move them to the hinges or use though-bolts (carriage) like the rest of the fasteners.