If your shulker boxes still look like standard vanilla blocks, appear in your hand instead of on your back, or fail to show the 3D bag shape, your game settings are likely missing the required custom model options.
Enabling Custom Model Features
This resource pack relies on OptiFine custom model features to show the bags properly in your world and on your character model.
1. Make sure your game profile runs OptiFine, then open Options > Resource Packs.
2. Confirm that Shulkerboxes to Backpacks Resource Pack is in the active right-hand column. Move it near the top of your active list so other resource packs do not override its shulker box textures and models.
3. Return to the menu and open Options > Video Settings > Quality.
4. Find the custom model toggles in this menu and switch them ON.
5.
Checking the Fix
Open your inventory menu to verify that shulker boxes now show their updated travel bag sprites. Next, place a bag into your offhand slot and switch to third-person view, or place one onto a block in the world, to confirm the 3D backpack model renders cleanly.