question about bass trap

Hello there, I have a question regarding the design of john's bass trap. Is there a particular reason why there is space between the cloth and the outer insulation ? Is that just roughly illustrated ? Should not I put it as close as the cloth I can ? Is a particular distance between both insulation or between the inner insulation and the corner of the wall to respect ? Any advice would be appreciated. Thanks. Alex

could you link to the drawing in question?

http://www.johnlsayers.com/HR/index1.htm first drawing. Thanks