Yes, this can work, but the shape matters.
Here's a good way to think about vertical antennas:
Cut a round piece of paper. Imagine this represents the signal seen from above,
radiating in all directions.
Now fold the paper in half into a semi circle. If you put a flat reflector in back of the
antenna, this is effectively what you've done - you've doubled the signal strength
on the side opposite the reflector. Doubling the power is a 3dB gain in power.
This is represented by the paper model in that how the folded paper is twice as thick.
Now fold the paper in half again, so that it is a quarter circle. The paper is four layers
thick, and the signal has four times the energy. (Another 3dB gain is 6 dB total).
You accomplish this by using two reflectors that meet at right angles - just like the paper model.
Now, if you use a curved reflector, the actual gain depends on the particular shape. If you make
it a parabola, so that the the antenna is at the focus, that it becomes like a flash light, and
the gain can be quite high.