Rain at an angle will see the aperture as smaller than if it were falling straight down into a perfect circle. A rain fence some distance from the gauge will affect the angle of rainfall and improve the accumulation accuracy. The rain fence most definitely decreases the updraft winds near the gauge cylinder that tends to blow the rain up and away from entering the aperture.
Different subject: R M Young makes a siphon rain gauge 50202 that works well on boats/ships and buoys. It actually prefers a moving platform, not requiring the gauge to be level and stable. It has no mechanical moving parts. Only the water moves through it.