Do you know anyone who can weld? How about a bit of pipe, with a reducer on one end, with some rebar welded onto the side about two inches down from the top? The reducer would keep the umbrella from poking all the way through, plus allow water to pass through. I would image that you could then get some sort of grommet to put in the top to cover it.
I just looked around and found alot of them that go inside of a doorway to hold your rain umbrella's.... I will probably just make some. You could maybe even use a piece of PVC with a cap on it that would fit down in the hole to fill/cover it, put a hole in it so you could hook it to pull it out.