I'd rethink the padding idea if I were you. In the lightweight packs I make, which I and others have found comfortable up to about 30 lbs, all of the padding - shoulder straps and hipbelt - weighs a little less than 1.5 oz., and I'm confident it's more comfortable than the clothing idea would be.
Size-wise, everyone has different requirements, but you won't be too far off if you make something that will hold around 3000 to 3500 cubic inches - assuming you have fairly compact gear. You probably have some sort of duffel bag or suitcase or something that you can put all of your gear into as a test - then figure out how much smaller or larger than that your pack needs to be.