ItemRegister class:
public static final RegistryObject<Item> ARMOR = ITEMS.register("armor", () -> new ArmorModel(ArmorMaterial.DIAMOND, EquipmentSlotType.HEAD, (new Item.Properties())));

ArmorItem class:
public class ArmorModel extends ArmorItem{

    public ArmorModel(IArmorMaterial p_i48534_1_, EquipmentSlotType p_i48534_2_, Properties p_i48534_3_) {
        super(p_i48534_1_, p_i48534_2_, p_i48534_3_);
    }

    public BipedModel getArmorModel(LivingEntity entity, ItemStack stack, EquipmentSlotType slot, BipedModel defaultModel) {
        custom_model model = new custom_model(1.0F);
        return model;
    }

    public String getArmorTexture(ItemStack stack, Entity entity, EquipmentSlotType slot, String type) {
        return "mymod:textures/entity/magic_hat.png";
    }
}

custom_model class:
public class custom_model extends BipedModel{

    private ModelRenderer bb_main;

    public custom_model(float size) {
        super(size, 0, 64, 64);
        texWidth = 64;
        texHeight = 64;

        bb_main = new ModelRenderer(this);
        bb_main.setPos(0.0F, 0.0F, 0.0F);
        bb_main.texOffs(0, 0).addBox(1F, 1.0F, 1.0F, 4.0F, 9.0F, 3.0F, 0.0F);

        this.head.addChild(bb_main);

    }

    public void setupAnim(Entity entity, float limbSwing, float limbSwingAmount, float ageInTicks, float netHeadYaw, float headPitch){
        //previously the render function, render code was moved to a method below
    }

    public void renderToBuffer(MatrixStack matrixStack, IVertexBuilder buffer, int packedLight, int packedOverlay, float red, float green, float blue, float alpha){
        super.renderToBuffer(matrixStack, buffer, packedLight, packedOverlay, red, green, blue, alpha);

    }

    public void setRotationAngle(ModelRenderer modelRenderer, float x, float y, float z) {
        modelRenderer.xRot = x;
        modelRenderer.yRot = y;
        modelRenderer.zRot = z;
    }
}