I've got one of these - which I love. However, when riding with a full cup, it rattles enough that coffee makes its way up through vent hole onto the top 'shelf'. A few more bumps, and the coffee can make its way over the lip - which wouldn't be a big deal if it wasn't located directly in front of my crotch, with a constant headwind

Maybe I'll try the neoprene idea - that ought to help.