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

For experimental purposes (trying to find the correct internal volume reduction for a given sound) you could cut off various lengths of appropriate sized blocks of wood. Easy to do and to dial in to your preferred sound. Once the amount of volume reduction is known, you can choose a more permanent final solution, if you like.

If you use Flex Seal, it may have petroleum distillates in it, as the more expensive rubber sprays at Wally World (10-12 bucks) will have. However, the cheaper stuff (2-4 bucks) doesn't have pd's. Pd's are rather corrosive to electronic circuitry.

Automotive undercoating spray (same thing as Flex Seal really and is what you'd be looking for at Walmart or your hardware store) will dry overnight to roughly half the thickness at which it was applied. However, after several days, the depth of the coating will continue to shrink and then be no thicker really than a coat of paint, so it might require simply too many coats and too much drying time to be of practical use as a way to reduce volume on its own...but good as a sealant, though. 

Flex Seal could possibly be thicker, but I have no idea if it contains distillates.

Styrofoam, I would think, would not absorb too much, although something that low mass may be excited to resonate a bit maybe, coating it with the rubber spray may help somewhat. 

Any object left inside the cabinet must Not be hollow, fill it with sand, if nothing else!