The Large Rucksack allows you to have nine slots and increases the backpack capacity in MW3 Zombies. This means you can keep more items. There are multiple methods to get this one and to know them, you must continue to read this guide.
Complete The Contracts
So, completing the contracts is the first method to get you the Large Rucksack. But remember that it will not be guaranteed, and obtaining the Rucksack will be completely random in this case.
From Buy Stations
This one is the most reliable and guaranteed as you can purchase the Large Rucksack from the Buy Stations in MW3 Zombies. But the condition here is that it is only available at the stations in the Red Threat Zone, so you will have to consider the difficulty level of enemies before going there.
Besides this, it costs you a massive amount of Essence, which is 10,000.
The other method you can try to get the Large Rucksack is looking for the Aether and Loot Caches. Both of these can provide you with the desired Rucksack. For the Aether Caches, you must visit the infested buildings, while for the other one, you can explore the simple buildings you may come across while wandering around.