Actually egg cartons REALLY do work well, at least in a pro audio situation.
When we converted half of my garage in England back in early 80,s to a practice room/ recording studio it always sounded hollow and harsh.
I read somewhere at the time of applying egg cartons to walls and ceilings for acoustical treatment on the cheap.
I used to get the wholesale size 24 count square trays not the carton. 4 large head short roofing tacks and it did not take long to fully line the walks and ceiling.
What a massive transformation!
I still have some demo cassette tapes we cut in that room and they still sound good to me.
So although that poster may have been joking( I hope!). I can state they worked wonders in our situation.