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

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.

Wow! Thank you all for some really creative suggestions.

Questions though:
It sounds like the Black Hole 5 would be similar to adding wool, fiberglass, acousta stuff etc. material. These materials are used to absorb and to compensate for a smaller box, not reduce for a larger box. But, I will investigate.
How is the Cascade V applied?

Adding wood bracing inside is the best way to go, but the inside of the cabinet is curved with very little room to use tools inside.

How about PVC pipe sealed at both ends?
Or a cardboard box sealed on the outside of the box with that Black sealant spray that I see on TV?
Can the spray expandable foam be used? Or will that absorb? Perhaps after it is applied I could spray the Black sealant over it? But that would be messy and stinky for a while.

ozzy