Most open cell egg-crate foam, like cheap mattress toppers, works pretty well. Also folded polyfill for like blankets and other crafts (Hobby Lobby/Joanne/Walmart). Adjust amount to suit.
I know my first KLF-30s I tried the "more makes the box bigger" to try and extend bass. It killed the bass. Try differing amounts.
The bass wavelengths are usually too long so you wouldn't necessarily be absorbing the low frequencies but changing how the box acts and the reflected higher frequencies.
Many believe that the % increase of the box size is more with sealed vs ported cabinets and from the speakers I have built and tuned, it really depended on how the box and woofer behave as a whole.
An old pair of Paradigm Studio monitors I had (original before Studio 100) had barely any open space in the bass reflex cabinet. With that cabinet and Vifa dual 8" woofer combination, was very punchy and the extension trounced k-horns that sat next to them.
Measure and listen. See what actually works with the combo. Don't kill the life of the speaker trying to gain extension.