I made a book to hold chair plans . The cover is made of two pieces of posterboard sized panels of a board sold for sign making. It is a white lightweight board that is sold in school supplies. It looks like a sandwich of posterboard and foam. I "hinge" them together with clear packing tape. Tape each individual plan to the hinge end with same tape. Been using it for two years now and it works great. My book has about 20 pages so far.
Where do you get your posterboard? My wife buys 32X40 matboard material for matting her watercolor paintings. They come in very solid, thin boxes. Maybe your source would have some of those boxes.
I guess it depends on how much protection you want to provide. I made this box before I left Alaska to protect several thousand dollars worth of unmatted lithographs. Internally it has foam top and bottom to keep the contents compressed while the prints themselves are in acid-free envelopes. Clearly your materials won't need this much protection but this enclosure was easy enough -- just some scrap pine, tempered hardboard, and hardware store corner brackets, hinges, and handle. and probably only took a couple of hours to "design" and build.