Now that you mention that, I could see a mod being made by attaching some sort of acrylic strip ( being waterproof ) across the entire openings to hold the bottles in place. And they would most likely be secure even while traveling. Or possibly a netting material would work as well.
Anything solid across the end will trap water, unless it's up a bit and leaves an opening at the bottom for the water to run out.
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