By the way, I don't have the 100 Geez in my pocket just now. BUT....
If we could come up with a realistic design (not egg cartons thank you)
that would have actual values for sound attenuation over the full acoustic spectrum) and would meet safety requirements and provide ventilation, then I do believe it would be a viable proposition.
If the "bunker" could be broken down into a flat pack that would fit into a shipping container then it could easily be shared between festivals.
Thanks Sorcha, I am thinking along the lines of 8 x 4 Ply sheets also but I think that we need something really good in the sandwich, possibly a thin sheet of lead (do not eat!) with foam either side.
Air locks or sound baffles on the entrances would be essential due to the "weakest link" principle.
It would of course be easier if the venue was further away from the noisy stuff but that is often hard to achieve.