Basically, why not make a mod who let the player to equip a shulker box on the "second chest" bauble slot.
I mean this slot circled in red.
It could be rendered or not on the back of the player, and a hotkey could be available to open it directly (without placing it on the ground).
Reasons? Well, most of backpacks mods here are outdated and/or have so much complex (and not vanilla-like) behaviors for a simple portable storage mod. On the other side Shulker boxes were introduced by Mojang on vanilla, and i totally see them as a makeshift backpacks if a mod enable them to be equipped on that bauble slot.
Think of this mod somewhat like Bauble of Undying, a simple mod who add a small but noticeable feature on a vanilla item to a great API.