I used cardboard and covered with fleece put rod in middle like rod with book from avenue q but it is a little heavy. maybe put dowel on end?

thinking of maybe using posterboard instead of cardboard any ideas?

Could you use 1/4" foam and cover with fabric. Perhaps make the spine of the book with poster board.

It is more likely it is the fabric that is adding the weight more then the cardboard.   I would make it out of foam core which you can get at the hobby or art store. It comes in colors so you wont have to cover it.

I love this book, though!

They're really nice! I did some books, too, but of course they are much smaller (about 2-3 cm, 0.79-1.18"). I've cut out some simple paper (cheap computer paper's fine), sewn it together with a few stiches, taken a bit of thin cardboard, covered it with fabric (glued on)and glued the first and the last piece of my computer paper to the insides of my book cover. But I guess for something this big you'd need an awful lot of paper and you'd have trouble sewing it together (Paper's darn hard to sew!).