Here's a thread where a guy put a beach ball under his slide awning. I would think it would also work if you have a regular awning over the slideout.
I guess I'm off to buy beach balls!
Here's another where someone had a PVC thing he put on top of his slide.
Awning problems on slides
Search for "awning slideout beach ball" and you'll get others.
1988 Coleman Sequoia - popup (1987-2007)
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10