I've built three of these so far, feel free to message me in game. Even with them knocking natural mob spawns down by almost a factor of ten, a properly build guardian farm should still yield visible spawns every minute, wish that could be said of creeper/skelly/zombie machines but I should not digress.
Only necessary rules are as follows:
Mob spawns must be enabled, check with /plot perm, toggle with /plot toggle mobs.
The grinder must be build within the confines of the original monument, usually thats y= 38-63 in a 58 block square footprint coinciding with the temple floor.
The grinder must utilize source water, no flowing water will allow gaurdian/squid spawns.
The grinder should have a lid on it at 63 or 64.
The last one many overlook: You cannot be within 24 blocks of an area previously considered valid for spawning, your presence will prevent the spawns, fix this by making an AFK spot in the sky, or digging down lower. Remember that certain entities don't even remain rendered/calculated past 64 blocks(this is a server limitation not mc), if you're going to make any non-hopper item transport system, this must also be close to your afk'er. You can remedy this by running a hopper-only setup.
Once again feel free to message me in game about this or any other grinder setup, I am well versed in the field of minecraft mechanics.
