Insulating slab garage door
I would use 1" or 2" rigid foam. You can glue it, but I would glue and use small bolts and fender washers on both sides of the door to keep the bolts from pulling through. #8 screws or carriage bolts should both work. Carriage bolts would look better. I would use locking nuts also. The foam's R-value is not as high as I would like.

Perhaps another alternative is spray foam--but I am unsure how that will work on the door seams.