If your backpack's external frame died years ago, you can easily make one with around $US3 worth of PVC piping and fittings. The only tool you'll need is a PVC pipe cutter.
Even though many backpackers have moved beyond external frame backpacks to fancier (and much more expensive) interior frame packs, budget minded hikers and veterans tend to still use external frame packs. YouTube user GaryBriggs3 came up with a great way to make a DIY frame for military ALICE backpacks or other external backpacks.
He bought 3m of PVC, four T-connections and six elbow connections. He cut the pipe and assembled the fittings into shape and then put the frame his oven at a low heat for a few minutes to make the PVC malleable so he could bend the straight piping to make it a better ergonomic fit, and made a simple jig with three vise-grips so the frame would cool and harden into the ergonomic modification.
The video gives you the basics. If you want more photos and info check out the source link below for the full writeup at the Zombie Squad forums. Before you click play on the video keep in mind that the author uses a decent amount of profanity if you're bothered by that.