1.21.5 Custom Model Data with Custom Item Texture (Hand Held POV)

What would the .json file (in the "Items" file of the resourecpack) format be if I want a Custom Model with a separate Texture (png) for a Custom Item model for this model when handheld?
For example, I have custom model hat that is 3d, but I want the item in the hotbar and handheld to be a 2d texture