So. First of all, you need to locate .vmt material file of the balloons.

It's located in this directory:

SourceFilmmaker\game\workshop\materials\models\robgamings\austinthebear\fnaf2\balloons

There, you can find a lot of textures and materials, made specifically for the balloon models.

Now, open any .vmt using VTFEdit(can be downloaded here https://valvedev.info/tools/vtfedit/ )

You can use simple Notepad for editing materials.

In the text, find $translucent line.

As you see, it's set to 1. It means it's active.

Now, set it to 0. Now it won't be active anymore.

Change every .vmt file, typing 0 in every $translucent line, except WHITE.(white is used for balloon ropes)

Now you're done! Launch SFM, open map, and see the results!