There's a thread here where a guy made a pack out of paper mache and card board. It came out ok. Probably did not last long. Packs take a lot of punishment. I made a pack out of pep files (I no longer have them, and if I did I promised not to distribute them.) but if you search the forum, a builder took Stephen's plans and used them as a pep template. That pack came out pretty good.
I just wondered. I have a shell pack to work on. But I really don't care for the prices for the replica parts. Since I have built a budget pack once already...I just wish I could find a place that has cheaper parts available if anything..
We built a mini pack for a friends son for Halloween. After making the shell out of foam board, (with wooden supports and backing on the inside and lots of wood glue on the outside) we did cover it with paper mache. Let's see if I can find the link but here are a few photos.
Mini pack beside my fibreglass shell. Probably would have been better if we coated it with a resin but it's held up pretty well so far. It has been a couple of years.
I've made 3 packs and the traps that my team wears from on our belts from EVA foam, the foam floor plates you put in your exercise room or laundry room. I love the stuff. It's light weight and durable and you can build nearly an entire pack from it. Plus 15-20 bucks gets you like 6 large sheets of it.