Just a thought since I never owned a volleyball net and it may be a non-issue. I'd be concerned about someone grabbing the net and either breaking the poles or ripping the net. Seems more likely in the water. How about fabricating something to mount the poles on a couple of those beefy springs that are used for mobile antenna mounts? You don't see them much on vehicles anymore other than some ham operators but up here they use them to mount fiberglass poles on fire hydrants so the hydrants can be found when covered with snow.
Al