Let me preface this thread....
I did search before I made this post.
I have a very good source of a lot of used salvaged appliances, being sent to scrap...
What would be wrong with using the sheetmetal from washers, dryers, and stoves for covering the bus windows that you want covered/blocked out?
Once it's painted, you don't know what the metal was anyway. I'm referring to enamel painted or powder coated sheet metal, not porcelain.
An angle grinder would make quick work of the metal, and you could grind spots for plug weld holes.
If CHEAP is good, isn't FREE even better than good????