You have to be able to walk 100 meters while holding onto the backpack, it has to be strapped to your back, and it has to be a commonly available backpack of a variety purchaseable in a store with the next 25 days.
No part of it can touch the ground, nor can anything attached to it other than your feet and shoes touch the ground, and everything that you have must be fully enclosed by the backpack, and you must cross through the portal on your own two feet.