In regards to waterproofing, I make similar sheaths, mainly for larger blades like bolos and such which you buy that never have sheaths. You can make them waterproof. After you lay down the templates run a bead of 5 minute expoxy or similar against the one that the edge will make contact with and then duct tape right down over the template. Press seal it against the bottom and then work it over the top. There is no cardboard exposed if you look down in the sheath or anywhere else. If you soak it long enough you can get water to seep in around the tape but it takes awhile. Just take a square of cardboard and wrap it in tape well and see how long it takes for water to get through the tape. On a really silly note they are better than kydex for extreme temperatures because Kydex cracks when cold and deforms when hot and the cardboard ignores both.
-Cliff