Hi,
Up till yesterday when I had an "open to below" area with an open rail I needed to create a solid to represent the "nosing" the stair installers added to the edge. I am working on a design that has a lot of "open to below" space and discovered an easier way. Instead of turning off the bottom stair rail like I normally would I left it on, dropped it to "0" height, made it wide enough to overhang the drywall about an inch and the same thickness as the floor finish. The results are pretty close without messing with the rail profile and if I get really ambitious I just might make that happen as well.