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

Why not use the higher number as the goal? Actually you're almost there without doing anything. By the way, that should be cubic inches, not inches. 

In most cases there is way too much stuffing, fiberglass or whatever, in speakers. The best results will be obtained by removing most or all of the stuffing. Let the speakers breathe. Any high frequency ringing, etc. internally can be handled with a small amount of hollow fiber wool, about the size of a grapefruit. 🍊The worst thing I found with bass drivers was the dust cap ringing. That can be addressed with a small damper such as the Acoustic Disc from Golden Sound.