1. Take a backpack(1). Looks like it works for anyting - I could put Taloon into Taloon - but Mountain ones have the greatest effect)
2. Fill it with stuff
3. Find a Hunting/Mountain (30/35 slots) backpack(2)
4. Put (1) into (2). It will take 25 slots no matter of how many items inside of (1)
5. Fill the rest 5/10 slots with stuff
6. Find another Hunging/Mountain backpack(3)
7. Put (2) into (3), the procedure repeats.
8. Fill the rest 5/10 slots with stuff
9. In case of three Mountains, you can have 35 slots inside(1), 10 more in the rest of (2) and 10 more in the rest of (3), 55 items in sum. Neat, huh?