Humidity is based on surface area of the water, not the amount. Think of it this way, 8 ounces of water in a tall thin champagne glass will produce alot LESS humidity than 8 ounces of water in a bowl, spread out.
You dont have to use the channels in the incubator, you can use bowls, cups, and even sponges to adjust to get it just right. Put them in areas where you can get to the with a straw thru a vent hole, then you never have to open it to re-wet them.
You will need to consider lockdown separately, because you don't want open bowls that the babies can get into and drown, but through incubation, find a combination that gets it where you want it.