You can take a few rockwool cubes and brake them apart to fill the bottom of your cups but wet them first because you shouldn't breath any rockwool particles.
That's the reason I use only
grow rocks.
Because rockwool acts like a sponge I don't think the water would have to rise very high to work My guess is about 1/2 inch up your cube.
With grow rocks the height should be about this high.
Ron