Whatever you add should probably extend the length of the port. Just sticking a random piece of wood in the port will probably
show a decrease in tuning simply because of the added resistance to the port air plug movement, but it will also hurt port efficiency and will be entirely unpredictable.
Also keep in mind that reducing port area will make the port less efficient and lowering tuning will do the same, so I would expect a lower score on the meter. Less efficient ports will get you more cone movement though so it might help a tiny bit with cooling, but I doubt it will be more than a degree of difference.