Consider: one member of your party is carrying a bag of holding. You sit inside it: not all the way inside it, just with your arms and head poking out of it so you can swing your weapons or cast spells.

A large portion of your body would be safe inside the bag, untargetable by enemies, effectively giving you as much as +5 to your AC and dex saves.

Someone tell me why this wouldn't work?

Scenario 2: a spellcaster casts "fly" on himself, then he steps into the bag of holding and pulls it up to his armpits, then straps it over his shoulders. He can fly around as he pleases with the vast majority of his body safe from attack.

Scenario 3: A gnome battlesmith artificer has a bipedal steel defender he's been using as a "mech" mount anyway when he gets this idea. He uses one of his infusions to make a bag of holding and he builds it into the mech where he normally sits, putting most of his torso safe inside a pocket dimension and massively boosting his AC.

I have to assume someone has already thought of this because it seems obvious in retrospect. Someone please tell me why it shouldn't work so I can get on with my life. Thank you.