I couldn't find the rigid board so I bought the densest attic insulation I could find and stuffed it into Quik-Tubes (for concrete pillars) These are the results.
-----no tubes------with one tube---with two tubes-------------
1K------------71db -------71db -------71--------------------
200Hz--------73db -------74db -------75--------------------
160Hz--------75db -------76db -------75--------------------
125Hz--------76db -------76db -------69--------------------
100Hz--------69db -------69db -------73--------------------
80Hz---------69db -------73db -------73--------------------
63Hz---------69db -------74db -------75--------------------
50Hz---------69db -------76db -------68--------------------
40Hz---------69db -------68db -------67--------------------
31Hz---------73db -------67db -------70--------------------
25Hz---------74db -------71db -------68--------------------
20Hz---------73db -------68db -------65--------------------
-----no tubes------with one tube---with two tubes-------------
1K------------71db -------71db -------71--------------------
200Hz--------73db -------74db -------75--------------------
160Hz--------75db -------76db -------75--------------------
125Hz--------76db -------76db -------69--------------------
100Hz--------69db -------69db -------73--------------------
80Hz---------69db -------73db -------73--------------------
63Hz---------69db -------74db -------75--------------------
50Hz---------69db -------76db -------68--------------------
40Hz---------69db -------68db -------67--------------------
31Hz---------73db -------67db -------70--------------------
25Hz---------74db -------71db -------68--------------------
20Hz---------73db -------68db -------65--------------------