Oh I see - you want the portable thing for the session people. Yeah, I suppose that might not be a bad idea, but not as much fun as sitting around the camp site.

Jamberoo has sessions in the Youth Hall, and the National has the singing room for singers and the big session bar for the tune players. To be honest, I haven't noticed the kind of problem you describe all that often, and I wonder if enough festivals would be interested in the portable session room to make it economically viable.

In the UK I heard of a company that makes inflatable buildings, including an inflatable church and an inflatable pub. They hire them out. Seriously! I seem to remember a blue clicky on the "trailer trash" thread which took you to the site. Wonder if they have anything like that here? :-)

Jenny