Aquaseal sticks forever, but it is less stretchable/flexible and can be a little hard if Cotol 240 is used as an accellerator (shortening the curing process from about 24 hours to about 2 hours.) So use just enough of it to do the job and if practical don't use an accellerator.
The other option is a good grade of neoprene cement, but it sticks best to neoprene so removing the outer layer of nylon over most of the area to be covered by the pad improves adhesion - but adds another layer of complexity.
Back in the pre-kevlar day we would tape an outline for the "pad", or in cases where there was already an existing neoprene pad we wanted to beef up, we would mask around the pad. Then we would coat the pad area with a thin coat of Aquaseal to improve durability. To keep it from being slick (slick knee pads on a pitching and wet deck are not a great idea) we would sprinkle fine sand evenly across the area (a suitable salt shaker works great). Obviously you needed to proceed neatly and carefully to avoid making the whole thing look like a mess. But it was very effective even if you made it look messy - and it had the benefit of sealing any existing and hard to find pin holes.
