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

Cascade V bloc..Thick purple paint..sucked into the mdf..seals the contents of the wall material from the energy robbing glue and saw dust that kills the dynamics. This product drys hard like concrete.. You could add a quart of micro bearing steel to increase the amount of base solid content. Dont use sand, kitty litter, rice, or lead..Tom

Black hole is very effective. I use it in my Klipsch Epics and the improvement was very noticeable. Glad to hear about the GR Research No Rez. I will look into it.

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.

Fiberglass insulation!

I second Black Hole 5 - I used it on a DIY speaker project.  It is meant to go only behind your drivers for rear wall dampening.  If you put too much (like the entire internal speaker) you will rob the speaker of bass.

As Bdp24 mentions, it comes with a very strong adhesive backing.