Some suggestions (and I'll try to see what others come to mind later):
Home Depot sells rolls of thin aluminum sheets:
I bought a roll a few years ago, cut it into three pieces and made a solid back reflector. Works magnificently for one of my antennas. This aluminum sheeting is the same material as your kitchen foil but sturdy enough for the outside. And foldable.
You may want to place a sheet of it about 4 to 5 inches directly under the antenna to reflect signals onto it from below (probably 2/3 the size of your circular antenna). Perhaps even bend it on an angle from the railing under the antenna so it forms sort of a trough. See the picture of the antenna reflector(s) bent under the antenna here:
The reflectors pictured (the bottom and top reflectors) are called "trough reflectors" You might even make sort of a "V" or "U" with the foil to go under the antenna and over it but at angles that allow a signal through. It would resemble how someone at the beach uses a sun reflector on their face to direct the sun's rays onto their face by holding it under their chin:
An antenna reflector in effect acts exactly like a sun reflector - to direct and deflect electromagnetic waves onto an object (and the sun's rays are EM) from different angles. You might have to experiment to see what angles work so as not to block any signals.
BTW type in the words "antenna trough reflector" into Google and when the results come up, click on Images to see pictures of different types of reflector shapes that you may find useful (and aesthetic). Semi-circular shapes are also highly effective.