Attaching a bag to a rifle case with back pack straps

[Andy T]

I have a number of nice rifle cases (DTA, Sig) that have back pack straps and a good suspension system, but could use a little more storage.
Has anybody thought of a way to attach a small/medium sized backpack to a case that already has back pack straps (I don't need gunslinger 2 at this point).

Thanks!