Mountainsnith's Scream 20 is lightweight, very durable and fitted with some nice features such as the full length zip.
Mountainsmith says:
The swiss army knife of minimalist packs, the new Scream 20 converts from an ultralight day pack to a stuff sack for sleeping bags, warm layers, dirty clothes, and muddy boots. Weighing just 14oz, the pack is ideal for ultralight peak bagging and trail runs or as a stash pack for longer backpacking adventures.Tuck the shoulder straps into the backpanel sleeve and the Scream 20 becomes a compression sack with reinforced roll-top closure and compression straps for more efficient packing on and off the trail.
Simple but very, very tough! Mountainsmith's Scream 20 Litre Rucksack weighs as little as 320grams and is a basic design with some well thought out features. It opens completely with the full length zip revealing an internal zip pocket for small items and valuables. The shoulder straps are unpadded but usefully wide to spread any load and attach to the base of the pack using aluminium hooks.
There's an additional deep pocket that forms the back panel, this is designed to tidy up the straps when you want to use this pack as a stuffsack - you could add some foam for comfort or stiffening if you wanted to upgrade the pack for a particular activity, or you could stash waterproofs here, packed carefully they would provide cushioning from the contents of the main compartment.
The top is a simple roll top with two buckles that click into the corresponding buckles on the main body.