Ultralight packs save significant weight by dispensing with frames-the stays, sheets, and foam that traditionally provide form and cushion. Fortunately, you can easily improvise your own frame, improving load transfer and comfort. Here are three ways to do it.
>>> The Flat Pad Fold a mat against the inside wall of the pack behind the shoulder straps. Therm-a-Rest Z-Rest pads are popular for this method, since their accordionlike sections fold flat and rigid. A 3/4-length self-inflater, folded in quarters and slightly inflated, also works.
>>> The Barrel Use a regular closed-cell foam sheet (a Z-Rest won't work) or partially filled self-inflater to line the inside perimeter of the pack like a barrel. Pack the inside tightly; the result both vertically stiffens the pack and cushions the back.
>>> Padless Use two long, narrow stuff sacks densely packed with gear as side-by-side vertical stiffening pillars.