Nice idea with the pizza pan!
Having tried both the vertical and horizontal nipples, I agree that the horizontal nipples do dribble, too. But they dribble WAY WAY WAY less than the vertical ones. The first winter with the vertical ones, I hauled out no less than 4 massive chunks of ice (each chunk about the size of 2 large watermelons). Last winter, with the horizontal nipples, I hauled out ONE chunk of ice the size of a cantelope.
Installed properly, neither the vert or horiz nipples should leak (as in drip water all the time) but both will dribble (when in use by a chicken). Another benefit of the horiz nipples other than drier litter is since they don't dribble so much is you can have longer periods between water fill-ups. With the vert nipples, I had to fill the bucket about every 3 days, with the horiz nipples, I can go 5-6 days.