I used the foam that you get from Home Depot that they use to seal window air conditioning units. It’s a square cross section, maybe 1.5” square, so you need to attach it a couple inches from the bottom edge of the skirt. But, it squishes down nicely to form a pretty good seal.
RV6/2001 built/sold 2005
RV8 Fastback/2008 built/sold 2015