Oh this is right up my alley!
First of all I have some ?'s. What type of snails? I'm assuming you have something in the apple snail family by some context clues. You say you've seen them mate? Are you sure that's what they were doing? Sure you have a male and a female? Why are you in such a rush for them to lay the eggs today?
Okay. I had my snails for months before I got eggs. I had seen them mating for a long time as well. What triggers snails to lay eggs is a good, varied diet, as well as the proper conditions (which can be more complex). What worked for me was a combo of watermelon, grapes, carrots, and romaine and leaf lettuce. Try to give them something they've never had before, especially fruit. As far as the space above the water line, my big girl had no space above the water line. She crawled out of the tank and everything looking for a spot. She eventually laid her clutch(es) on the inside lid of the tank hood. She's gone back to that same spot several times. I think she picked that spot b/c of the humidity level. Snail eggs that are laid above the water need tons of humidity to hatch, which is why none of my clutches made it. Do you have a lid on the tank? Something to keep the humidity up? Try placing a soft fake plant at the top of the tank. Maybe tape it so that it hangs over the side of the tank and into the water. In nature snails tend to lay their eggs on plants that are growing out of the water, above the water line.
Have you been to www.applesnail.net ? That's a good site to check out.
Good luck with your project Ham!