I'm not clear on whether you're building your own pack to save weight, or to save money. If it's to save weight, I'm not sure you're really doing it in the most efficient matter.

The idea that you'll pad it with clothing - lots of it, from the sound of your post - isn't going to save you much weight, because of the clothing load you're carrying. If your goal is to carry the lightest possible pack, then you can eliminate pretty much all the spare clothing except for a change of socks and your rain gear: wear nylon shorts with a mesh liner (no changes of underwear to carry) and a synthetic T-shirt (no spare T-shirts to carry.) Also, get a polyester (not cotton) fleece jacket and, if you need them, pants - you can pick these up at Goodwill or Walmart or a Dollar store for under $10 usually. They will be lighter than a sweatshirt and sweatpants, and will dry a lot quicker if you happen to get them wet. (They would also make great padding, assuming you intend to take them on every trip.)

If your intent is simply to save money, then go ahead with your plan to build a pack. You might want to look at the Gossamer Gear website - at one time, they offered plans for building a frameless pack; I haven't checked lately, but they may still do so.

Good luck, either way.