How to set the default value (structure)?

[ruanun]

map-cfg:
  c1:
    b: true
    d: 2
    c: "str"
  c2:
    b: false
    d: 3
    c: "str2"

type Config struct {
	MapCfg map[string]Cfg `mapstructure:"map-cfg"`
}
type Cfg struct {
	B bool
	D int
	C string
}
func TestCfg(t *testing.T) {
	v := viper.New()
	v.SetConfigType("yaml")
	v.SetConfigName("test")
	v.AddConfigPath(".")
	_ = v.ReadInConfig()
	var c Config
	if err := v.Unmarshal(&c); err != nil {
		panic(err)
	}
	fmt.Printf("Parsed Config: %+v\n", c)
}

In the above code, since the Cfg is nested in the map, how do I set default values for the fields in the Cfg?
Because the key of the map is uncertain, so it can't be set by viper.SetDefault.
Currently, the default value is the default value of each data type.
DecodeHook doesn't seem to be able to do it, I just need to handle the default values, the rest of the data parsing and binding is still done by viper.