Hey there,
I’m not sure if that’s possible, but I also don’t understand what you are trying to achieve.
I would like to keep my app portable
What exactly do you mean by that? How is leaving files in a user’s system helps with it?
This app can download contents that going somewhere at the root of the app.
Do you mean a project is meant to modify the app in some way?
Keep in mind users have the options to run projects with the Itch.io app in Sandbox, so they are not allowed to interact with anything on the OS apart from the project files themselves.