Problems with 3D model Chestplate

In Blockbench,

• Click on File >> New >> Minecraft skin, change the pose to none.
• Now go to File >> Convert Project, and change it to Modded Entity. Delete folders: Head - RightLeg - LeftLeg.
• Delete all blocks inside RightArm - LeftArm ... ** DO NOT DELETE THESE FOLDERS **
• Go to the Body folder and create your desired backpack.
• Save texture and export model.
• In MCreator add your texture and model
• Your backpack is an armor chestplate
• Set model - Body - LeftArm - RightArm - Texture
• Modify properties and Triggers, then save and test.

This should be similar to the end result.

Why File >> New >> Minecraft skin? 

Because it includes the proper pivot points.