Haven't been able to get on for a few days but have done a bit of research in the meantime.
Thanks Guest Jim......that would be lovely but perhaps a bit small (4x2.85x2.2H) I notice that they do bigger ones though.
This site
sound proofing info
has lots of good stuff including
AT LAST! REAL LEAD! THE BEST! ACOUSTICAL DAMPING LEAD METAL SHEETING & Radiation BARRIER! An economical, extremely effective barrier to sound. It's the thinnest and most effective acoustical material known, so is good for installation in/on floors, walls and ceilings for maximum effect. They give this 1/64" foil material a rating of STC 21. It weighs 1 pound per square foot! Professionals apply it to walls, sheetrock/drywall panels with our heavy-duty adhesive. Also blocks radiation from nuclear attack!   

That oughta do it!

Any ways..the point is (in my humble opinion) that technically this could be done.
Questions are... how great a need exists?
How much would it cost?
Would festival organisers be willing to pay for the cost of hire for an event?
Considering the large amounts spent for sound reinforcement at festivals, would the cost of providing a sound reduced venue be acceptable?
For events held within walking distance of a pub or similar building there is obviously no need.
What I am thinking of is the great many festivals that have perhaps 6 marquees spaced closely together and the sound levels are very high throughout the grounds.
Even the Guiness tents which used to have sessions during the festivals have now gone over to an amplified stage setup which mostly features the same performers that are on the main program.