If your "cardboard" was in reference to my corrugated sign material, the material I meant was plastic. It seems that all of the recent political signs in my area are of a corrugated plastic. The structure is like the cardboard stuff with the two faces separated by a central corrugated layer, but it's all plastic. Check with a local sign shop and see if they stock the stuff. After elections you may be able to liberate some of the stuff from a neighborhood empty lot. Then check your local hardware store for thin wall brass tubing. There was some discussion of this process at BPL a year or two ago.
For the CF frameboard I made I made a mold of 2x4's cut to the shape of the aluminum stays for my (then) favorite internal frame pack. I laid a sheet of heavy pasteboard over this curve and stapled it into place. Then I coated it with melted wax (as mold release)and laid up several layers of CF cloth using WEST system epoxy. It worked great and my oldest son now carries the pack. Expensive though at about $90 for materials.