Hey everyone, I just got done designing and making a backpack for my heavy loads (up to 35 lbs regularly if I had to guess) and I'm pretty pleased with the results. I made it out of material I had laying around which just so happened to match my gaudy green insulated pants and bright blue MB thermawrap jacket! The back panel , bottom, straps, and hipbelt are made of 200D oxford nylon (4-5oz/sqyd) and the front panel is 70D silnylon (2.5 oz/sqyd). The frame is 2 1/8"x21"x1" bars of aluminum along with a piece of blue foam cut to fit under the shoulder straps.
I attached the shoulder straps by cutting the back panel with an angle where the straps go, putting the straps there and sewing it back where it was. The tubes where the aluminum stays go have an extra layer of oxford where the ends touch the fabric. the stay ends are padded with 6 layers of duck tape to prevent excess rubbing. I didn't have any spare ladder locks laying around so I made some out of an orange juice jug for the compression straps on the back. They are temporary, but kind of neat.
One design feature I am proud of is the bottom of the front panel (blue fabric) has a 2" deep cat curve in it so the bottom of the bag doesn't sag!
The bag weighs 9.5 oz, but I am going to have to trim up the hipbelt and shoulder straps because they are too long, so it should drop to 9 oz. The removable aluminum stays combine for 6 oz while the blue pad weighs in at 3 oz. I expect the blue pad to come in handy for a sit pad too.
I don't know if you can see it, but the cat curve is in there