I was looking for a way to keep an umbrella handy. After installing a Mag Lite on the driver's side seat rail, I thought the passenger side seat rail would be a perfect spot to keep a small umbrella handy.
I took some cut to length double sided velcro (we use this at work for securing cables in server rooms), cut them to be long enough to go around the umbrella, and installed some brass grommets in the middle of the velcro. I then used some flat head machine bolts and secured them through the existing holes in the seat rail.
This holds the umbrella nicely, keeps it off the floor and handy for when you need it.