In reply to Jonny Tee 69: I've got 2 aiguille sacks, the zyphur (45l) and the cirrus (33l). They are both well made and durable sacks. They are however still quite light weight, 1kg and 800g respectively, this is due to a fairly mininmalist design, with no fancy back systems, or techie hip belts or other useless gizmos.
The super coloir is an older version of the zyphur, it is heavier and more featured.
The also do a beg, called the stratos, which is 40litres and sound like it would suit you needs.
Aiguille stuff is mail order so if you want th bag adjusting, for example gear loops on the hipbelt, then they will be more than happy to do this for you.
Can't recomend them enough. HTH