Where are you from that you use A4? Maybe it’s just me (and I’ve been out of America long enough to not know), but isn’t American standard paper size Letter? Just kinda curious. Maybe I’m just seeing something that isn’t there…
As for your design, by “getting bent” I meant more that, if the bag’s supporting/base material was too soft, then if the bag could get “malformed” (for lack of a better word) and bend the books/pads out of shape. But seeing as you were thinking of using leather or something, that’d be much less likely to happen, if at all.
and nacho’s “modular hard shell backpack” seems like an interesting idea. just the look of it made me think of TMNT… and that’s a good thing, in this particular case…