My fishing glasses by Costa always come in a nice case, they may sell these separately. You could glue Velcro to it and sew a strip on your pack. I have seen some tactical styled cases that take the Alice type clips or have their own straps this would be fool proof almost. Most packs have an accessory strap or one for a water bottle that would be a good place for the case.
"When you disarm the people, you commence to offend them and show that you distrust them either through cowardice or lack of confidence, and both of these opinions generate hatred." Niccolo Machiavelli