Wouldn't it be too big with the biggest one? Anyway, I agree that it should popup with the F9 menu. Maybe make the bags open up alongside the backpack and then put the equipment packs where the bags usually are?
However, I kinda like the Pack not showing up when I press F9, because I won't be using all that spare armor and weapons anyway. And the weapons I'm switching between are already hotkeyed.
Plus, having the equipment pack be right under your paper doll (which, really, you're only opening to look at or swap equipment around anyways, or to check your gold supply) makes it more convenient when you're equipping manually.
Hm i preffere not having it appear when i press F9, because i know theres just spare armor in there, and the rest of my bags house picked up items, spare weapons.