Since this is a lightweight forum, I'll suggest something like these:

http://www.integraldesigns.com/product_detail.cfm?id=770&CFID=1711017&CFTOKEN=63456768

http://www.owareusa.com/ under daypacks

http://www.mountainlaureldesigns.com/shop/product_info.php?cPath=25&products_id=139

One of these should fit your needs. Some of them could do double duty as stuffsack for other gear in your pack when backpacking.

_________________________
dk