I would advise against using cardboard or plywood as a transverse hull filler material. The edges of both materials are not easily sanded and shaped, particularly the end grain of plywood and, in my opinion, the process involves a lot of needless work.
An inordinate amount of time and effort seems to be generated in recent times in efforts to overcome the inherent weaknesses of plank on bulkhead ("POB") and plank on frame ("POF") kit ship model construction. Here, again, the critical thoughts I once expressed concerning ship model kits which apparently resulted in my being banned from posting on that "other" ship modeling forum (which increasingly has become a "ship kit assembling forum" controlled by certain ship model kit manufacturers) come to mind. Just as IKEA's "knock down flat" furniture has come to dominate the low-end furniture market due to the economies it offers in terms of processed materials, CNC cutting, shipping expense, and warehousing space required, so also have "knock down flat" ship model kits and hence the near ubiquity of the laser-cut plywood bulkheads and frames for POB and POF (as distinct from "fully-framed") kit ship model hull construction. In both cases, the "bottom line" defines the design parameters.
The fabrication of a "closed hull" model ship (i.e., not depicting un-planked framing) is far more easily accomplished by carving a hull from solid wood or, far easier still, by stacking "lifts" (aka: the "bread and butter" or "sandwich" method.) Nor are many modelers and certainly no kit manufacturers constructing hulls the way small craft are actually built in real life, via the "mold and batten" method. It seems the interest in "packing" or "filling" POB and POF hull structures is only a return to "solid hull technology" via a tediously circuitous route that combines all the unique challenges of each alternate hull construction method into one, quite likely evidencing the accuracy of Roger Pellet's observation that, "Assembling ship model kits teaches one only to assemble ship model kits."
Filling the spaces between widely spaced bulkheads and frames and sanding the filler fair is only effective as a support for planking which must form a fair curve between the bulkheads as the modeler's sanding of the filler material is itself fair. "Catch-22" of filler pieces is that easily shaped filler material is also likely to lack the structural qualities that offer a suitable fastening surface, whether by adhesives or mechanical fasteners and many filler materials raise archival quality issues, as well.
I submit that where kit design fails to provide a sufficient structure for producing fair POB or POF hull, the better option, rather that some sort of solid filler material, is to loft and fabricate additional intermediate bulkheads and/or frames to define the shape. The narrow solid edge of a wooden bulkhead or frame is far easier to sand to a fair curve with a sprung batten with sandpaper glued to its face than to try to shape a much larger surface of material filling the entire space between adjacent bulkheads or frames.
