Overnight Luggage Storage in Tokyo

Hi,

Myself and two friends will be traveling to Tokyo for a week and staying at a Hakone traditional ryokan in the middle of that week. I had been hoping to take a day pack rather than take all my luggage (it would be easier), and was wondering where we can store our luggage overnight in Tokyo? I have heard that many train stations have lockers, but that the large ones are hard to get a hold of. We are using two separate airbnb's for accommodation (already booked) so it isn't possible for us to store our luggage there for the night, unfortunately.

Thank you for your help :)