MachineMan

A medium size (about 18" long) umbrella fits perfectly stored resting above either rear seat adjuster.
It's out of the way of rear passenger legs both on entry/exit and while riding in the back.
You can still adjust the rear seat front/back while the umbrella is in place.
I store mine with the handle pointed towards the vehicle middle so I can just reach back and grab it when needed.

go3Ds

We use that spot for the same reason. We used to have the larger rear side door storage on our 2010, but our 2013 only has room for a bottle of water. Glad we found the unique space for an umbrella as you did. Very convenient.

jimmymagic

There are also umbrella holders under the front edge of both the driver's and passenger's seat.