You will enjoy this challenge Mark.

The best place is in a position thats an enequal distance from every cylinder, thats assuming your ecu can determine knock per individual cylinder from just one sensor.

The second best place is potentially the "end" of the engine.

The third best option is usually the inlet manifold and this is the option that is at least normally the easiest, but also one of the noisiest.

The worst place is probably directly on the cylinder head.

Once its mounted, you need to create a frequency profile. I dont have any ECU screenshots to illustrate this, but the knock system should give similar data so your going to be looking at data something along these lines:




Then you need to profile the engine sound with some safe spark lead and create a "Known Good" noise with which to reference change from.

The Plex system actually makes this quite easy to understand, but ive no idea how its going to work in your ECU. You end up with a baseline noise curve from the engine (everything below the line) and this line becomes your known "Knock Threshold". Once this is created you can essentially tell the ecu that if it hears noise above that line from now on, its knock!

This plex stock image shows some.






Once its all done, if your ECU allows, you can even create a nice knock table from it with load and RPM axis.
You will probably end up with a table that looks something like this: This is normally the table used as a reference to create the borderline knock table on ECU's that actively control load. (Pretty much all OE ECU's of the last 10= years work this way).







"Accurate" knock monitoring is for me by far the hardest part of setting up an engine management system and to be fair, it can and almost always does take so damn long you cant realistically charge enough money to make the job worth taking on. Its complex, extremely dangerous to the engine and fucking infuriating creating a baseline audio profile that actually "works" properly.

In other words - Just the kind of job you like mate!

I realise you know a lot of this, but I figuerd if I was going to answer, I would add some data that useful for anyone reading, not just the OP.