For the J-Pak I started with a lot of measurements. I printed off every picture I could find of it and decided on the basic shape of the thing. With that information I measured off my friend and scaled the pack up to fit him. With that data I cut out several layers of blue foam that would form the core of the pack.
The layers were secured with white glue and toothpicks. When the glue dried, shaping began. I started with a retractable knife to get the basic shape then switched to a coarse grit sandpaper for the finer detail. Drawing the shapes I needed on the foam with a sharpie helped with maintaining the design.
Once the basic shape was completed I started slathering on Bondo. The Bondo actually eat the foam a bit and forms a hard shell around it. Since this was going to be used for a convention as well as worn on someone's back(which means it will get beaten up) I wanted to make sure it would be strong. I probably could have gotten away with just coating the foam in white glue for a smooth finish but it probably wouldn't have survived the con. I think I wound up using 2 cans of Bondo and a bunch of sanding disks for my dremel. I didn't have the time to get the thing totally smooth but I got it decent enough to pass the 10 foot rule.
I used more tupperware containers and some scrap plastic for the intakes. The straps were scrounged from some shopping carts and attached to the back with anchor bolts. The tube down the middle was made from 3/4 inch PVC. With all the parts on painting was finished and the detail were drawn on with a Sharpie.