light but stiff sheeting materials for increasing pack firmness?

HDPE plastic sheet is commonly used by pack makers.

http://www.usplastic.com/catalog/item.aspx?itemid=23869

Ryan

Thanks Ryan – thoughts on thickness?

HDPE works OK. My only objection is the weight. High Density is the correct name. Two carbon arrow shafts (at around 1oz) would weigh considerably less than the ~6-8oz the HDPE weighs and provide more stiffness in the vertical 18" or so that it would cover. Your sleeping pad will also provide additional firmness if you use the 1/4" pads commonly available. Or just lay it flat against your back if it is too large to fit.

I did much the same with my old Trek. I removed the 1/4" foam pad and sewed the bottom closed. I added a second piece of fabric around the upper and lower corners after slitting the rear open. I slipped in two arrow shafts on either side and stitched along them, pretty much making them permanent (though the foot would only get within about an 1/8" of the shafts.) This enabled me to carry about 30-40 pounds comfortably with no pack collapse. Once I added my pad (1/2"x20x10 in 5 layers,) this was really comfortable.

Alternatively, you can add some external pad keepers (1/3 length pockets made from some mesh) that lets you add your pad to the pack. A 5 layer, 11" wide sleeping pad will work for warmer months with your pack under your feet and a stuff sack for a pillow. 5 payers will support about 25-30 pounds depending on weather you use dimpled pads, plain pads or inflatables. Just check to make sure your hip belt still works since it does change with/without the pad. You can use the existing pocket for other things.

Kodiak,

I did some checking around and looks like 1/8" is the most common size. James is right though, it will weigh about 6oz.

Ryan

try to use a garden seedtray bottom sheet. I have green ones that are checkered plastic that is nice and thick. weighs 3.5ounces once the sides are snipped off and can be utilized to increase ventilation. im designing a backpack based on one 24" tray sheet with aluminum stays on the sides

Are you looking to gain stiffness or just shape? If shape is your only concern then use foam. If you want a bit of stiffness then HDPE is the way to go.

Good stuff at ebay

http://www.ebay.com/itm/4mm-White-24-x-36-4-pack-Corrugated-Plastic-Coroplast-Sheets-/171533868113?pt=LH_DefaultDomain_0&hash=item27f0371c51

Corrugated Plastic Coroplast Sheets