I bought a sheet of thin plexiglass at Hobby Lobby today, thinking it would be a bountiful and reliable substitute for the "packaging plastic" I used to make my Gelatinous Cubes.
While it is "doable," it is much harder to work with because it is so brittle.
I did succeed in making a cube, but it took a lot longer and it wasn't as fun to do. :(
Just an FYI. Keep looking for that scrap plastic!