I had a similar question but didn't want to post last night given how tired i was, running the risk of missing something obvious haha. But looking again, that seems like its just a 6 inch ruler, which would mean this is really downsized. Am I way off the mark? I only took mine down to about 910mm for my average build and found that seems to be the sweet spot; still menacing without dwarfing me.
I harden cardboard with a mixture of well-catalyzed fiberglass resin and about 20% acetone. Resin really won't set until acetone fumes off. The acetone helps the resin penetrate the fibers rather than just sit on top. Don't overcoat it. Excess resin doesn't add any strength and ruins your details. The resin is a lot harder to sand down than Bondo or spot putty so don't let it pool on the surface of the cardboard.
I use Bondo to fill in the edges and surfaces in my cardboard work. If you can afford the featherweight aircraft Bondo, it is a lot easier to sand.