Yeah, the extra panels attached to the outside of the enclosure. For the interior, an alternative to wooden dowels is threaded steel rods, inserted into the enclosure through holes drilled into opposing enclosure walls, each rod screwed into opposite ends of a threaded coupler. The rods can be adjusted until they are tight enough to prevent outward panel movement. A locking nut can be threaded onto each rod, tightened right up against the wall interiors, thereby preventing inward panel movement. The rods can span the height, width, and depth of the enclosure, one set for each pair of opposing panels.
When I built my 4 cu.ft. sub enclosures, I built them double-walled, an enclosure-within-an-enclosure. I left 1/2" space between the two, maintaining the space with 1/2" "ribs". I then filled the space with #60 Silica sand. VERY effective wall damping! There were very large loudspeaker enclosures so designed and built in the 1950/60's, before Acoustic Research's introduction of the acoustic suspension principle killed them off.