Is this the Celotex used for making bass hanger?

http://aknightcompany.com/pages/sheathing.aspx and size is 2'x8' per sheet?

That will work fine; so would vinyl floor covering, MDF, etc - main thing is it needs to be free to move (hung, not nailed to something) and wrapped with insulation. Does NOT have to be exact dimensions, you would choose dimensions large enough to fit the available space without touching walls... Steve