Is it possible to stack multiple boxes on a Modular Car Storage Module, as shown in the Plugin Icon?

It is possible to implement, but comes with some caveats that the plugin should ideally handle. For example, normally a module cannot be removed from a car while it has contents. Ideally the same behavior should apply when a secondary container is attached, but vanilla will not natively detect that container, so the plugin needs to account for it somehow. Worst case, we could probably settle for the extra container simply dropping a storage bag when destroyed.

It's currently possible to use the plugin configuration to add extra containers on top of the storage module, by looking at the config for other vehicles. However, I don't know what problems you may encounter. I suggest that you explore this direction on a test server and report back how it goes.