Ok Starting to get a bit more of an understanding now (not everything, but some) so a couple of what if questions.

First if we assume sealed box

With the current design 28l and the cavity going down through the column, would I be better sealing of the top and making that a 20l (appx) enclosure and making the column and base solid.

in either case should I fill with rockwool.

for the upper cavity would it be better to have straight internal surfaces rather than curved , this would lose some of the volume (about 2l) so should I also increase the size to get the volume higher or would 20l be enough

Regards

Ian