To everybody, thanks for the support and compliments.
Anyway, changed my mind on the overflow box.
Tested my new design based on the Bean animal overflow last night. Using a Water Blaster 5000L/h pump. Still need to redo this test using the DC12000 pump to see if it can really handle big turnovers.
Using old bucket.
The pipe on the right is the one supposed to run full siphon. I did add a ball valve after these pictures were taken to get it right. Using soil vent valves as Durso pipes. On the right there are 2 holes, closed up with a bit of silicone for the test. Seems I need to add two airline tubes to get it right. The amount of air via one pipe does not work.
I suspect that using a 12000L/h pump will force another full siphon standpipe. If I can recall correctly, somewhere it was mentioned that one 50mm pipe can handle around 6000L/h.
I did add the ball valve, and it was really quite. Impressive, taking into account that it is 5000L/h running.
Just need to play a bit more to see if I can get the airline right, and that it does start correct after the pump was switched off. it does seem to take a while to flush the air out of the full siphon pipe and to stabilize.