Sorry its a bad picture and its smaller clones in water bottle hum. domes. But same idea. When i do it in solo cups i sit the second one on top carefully fitting the clone in the cup like a lid. Then right at the lips were they sit together i wrap duct tape around 2-3x’s
Then i stand em in a box that fits like 6 cups at a time in bottoms of my bookbag and off i go. I use a razor blade to cut the cups apart.
As for gravel in bottom. Yes it should be mixed in your medium/soil too. But i also put it as a layer by its self at the bottom of the cups.That layer under the soil helps with drainage but it also helps plant not get “as root bound” as when the roots grow straight into the bottom of the cups.
Hope that helps?