Transfer model to sd card

Have a nice day
In the demo, the model is in apk. I have an idea to download on demand a zip to an sd card and put it there.
Then I want to do something like this
```
File sdPath = Environment.getExternalStorageDirectory();
sdPath = new File (sdPath.getAbsolutePath() + "/model-android/");
```
but it does not work.
How to make loadable models not stored in apk?
Thanks for the example.