Best way to decrease the internal volume of a sealed speaker?

The foam will work, but it expands a lot. If you go that route, use the window and door foam. It is minimally expanding. But it does make a mess! Clean any up that gets on the outside of the cabinet immediately. You’ll need paint thinner for that. It should be acoustically inert, as long as it doesn’t break free..

What about cutting pieces of MDF that equal the volume reduction you require, gluing and screwing them together, then gluing them onto the bottom of the cabinet? Wood glue will be stronger than the mdf. You could also use dynamat.. I put some in my speakers to increase bass clarity and reduce sidewall resonance, but in a small speaker like that, it will definitely decrease the internal volume a relatively significant amount.

Sorry for the late reply.. the expandable foam will dry with a smooth surface, unless you mess with it. It will be flexible, though, so take Erik's advice into account. I'm not sure how flexible materials would effectively increase the internal volume, but Erik doesn't tend to talk out his butt and has much experience building speakers.

That said, nothing wrong with experimenting.

If you do use 2x4's, considering using blue tack to hold them in place until you're happy with the sound. Then, if the cabinets are unpainted MDF on the inside, consider wood glue. It is very strong when gluing wood together (duh) and would not be flexible like silicone, which could add too much damping. Or maybe you need the extra damping of the silicone.. I suppose you could start with silicone and, if it's over damped, cut it out and use wood glue.

By the way, those cabinets look quality. Let us know how it all turns out

Good to hear! (See what I did there?)

Synthetic wood was probably a good idea.. denser and not affected by humidity. Well done