Since this is a resource pack rather than a gameplay mod, you do not need any special armor slots or custom menus to wear a travel bag. Any standard or dyed shulker box automatically functions as a wearable backpack.
Equipping the Bag
1. Craft or grab any shulker box from your storage. Notice that its inventory sprite already looks like a travel bag.
2. Open your inventory (E by default) and drag the shulker box directly into your offhand slot next to your character preview. Alternatively, hold the bag in your main hand and press your swap-hands key (F by default).
3. Press your perspective key (F5 by default) to switch to third-person view. Instead of holding a block in your offhand, the backpack model appears directly on the back of your character.
Accessing Your Items on the Road
Wearing the bag in your offhand is visual unless you pair this pack with a separate storage addon like Advanced Shulkerboxes. In standard vanilla play, press F to move the bag back to your main hand, place it on the ground, right-click to access your gear, break it to pick everything back up, and press F to put it back on your character.