I use a pad pocket made of 1.8 oz rip stop in preference to the rolled pad. In my experience the rolled pad makes the pack ride somewhat like a beer keg with shoulder straps; not particularly comfortable IMO. My folded Thermarest Prolite 3, short, makes a good stiff "frame" if I go with the stay removed

It is better to plan for the stay while making the pack rather than trying to retrofit. I sew in sleeves for the stay and have a webbing pocket in which to seat the bottom ends. The pocket is sewn on to the hip belt reinforcement piece near where the hip belt attaches. All this is hard to do once the pack is assembled.

I got the aluminum from a metals supplier in Tucson. I don't recall the alloy number but it is used in aircraft. It needs to be bendable but still stiff. I used 1/4" diameter rod. The piece I use for the stay weighs a bit under 4 oz. If I can remember, I'll look up the alloy number but if someone else reading this has the info, don't be shy about posting it.