After experimenting with the above design, I find that to operate a siphon with lower water inflow an elbow is required as in photo below. This outlet shown below is making use of a 25 mm to 20 mm reducer which makes it more stable and reduce noise as well.
This particular outlet style I don't recommend any extension horizontally after this elbow it will cause the siphon slow to cut off due to water lock within this elbow to final water out. What happen as the elbow is during initial siphon starting, water drop down the tube will hit the elbow ad this cause temporary seal, gating the water out and at same time draw air out from above. As the air depleted from the siphon bell it will cause a flush since the gating effect will now draw water downward.
The 25mm to 20mm reducer is just to ensure the siphon is more stable at lower water inflow. Using a 25 mm tube all the way on this arrangement require more inflow, since "elbow gating" will require more water to creates.
This is a closure video of my Straight Outlet that I was experimenting few months ago. It was tested extensively and I find that though it work but it requires more water flow to start. This will make it dependent on water inflow.