as someone who does a cosplay for a character with a backpack and a big one for that matter i would highly suggest adding some sort of strap with a buckle (if you want it to be removable while wearing it) at cons there is such a high probability of something running into you or bumping it on something that without a ton of magnets holding it on its very likely to fall.
This backpack is bolted onto my backplate, but I have a strap that runs from my backplate to my belt in the front, which also acts as a sort of harness and leaves less stress on the back after a long con day