too large box

[Peter M.]

Or emty bottles with no cap on.... Seems like an easy way to make helmholtz resonators.

[Samuel Jayaraj]

Make a box within the box and fill the gap with whatever you want. You can do this one side at a time. Only the speaker and connector cut-outs needs some care. You will end up with an inert box. Dynaudio use this technology for all their top models.

[Westerp]

I usually put bigger speakers in them to get the volume down & up