Looks like you'll want something like this.
Though I will say if you're getting significant sagging with the 18-35 1.8, a cheap ballhead/tripod is probably the problem. These lens supports are meant for even heavier lenses (think telephotos, like the old 80-200 2.8 which had no collar). What tripod are you using?
/u/wanderingangus - you might want to take a look at the $66.49 Manfrotto 293 Telephoto Lens Support.
The 293 is a solid piece of metal and more robust than rails, so it should handle active videography somewhat better than your current setup.
Hope this is helpful and good luck!