Something that Holzer writes in
Desert or Paradise seems worth mentioning here. I'm not sure if this is the same as the "collar" idea, I think it isn't. it's to prevent fish from escaping down the pipe (if you don't want them to go down it, I gather).
quote:
Every monk needs a safety device to prevent fish swimming through it. Conventional monks have a slit-plate installed before the dam planks. For the Holzer Monk I use the pipe-in-pipe system. I slide a pipe that is about 5cm in diameter larger over the vertical pipe. I use an angle grinder to make the slits and waterproof the end with gauze or builder's fleece. A spacer keeps the pipes in place. The pipe with the slits
should be at least 10 cm longer than the viertical pipe. This prevents fish from swimming through and allows sufficient water flow. This also helps to keep leaves out of the drainage pipe. (p. 73, Desert or Paradise,
Sepp Holzer)
Note--it seems the horizontality of the opening isn't sufficient to keep leaves from getting into the pipe (perhaps clogging it?). And this will work even when the Holzer monk is in a lowered position, not just when it's at full height.
Does anyone know why you need to waterproof the end? isn't that part always going to be out of water? I understand making it fish-proof, but not sure why a screen wouldn't be sufficient. Nor how gauze makes something waterproof...
Also, there's such a thing as a regular monk, and a Holzer monk. Regular monks are adjusted by removing some planks from the stack of planks, and Holzer's innovation (that someone else seems to have invented also) is to make the L-shaped pipe that can be adjusted.
Paul Wheaton's trickle pipe is another interesting twist:
https://permies.com/t/33162/Paul-Wheaton-trickle-tube#1236175