In my experience you will want to have the sensor flush against the case, not on top of the shroud. On VWs the knock sensor has to be flush against the block and installed at a very specific torque value to "hear" the right harmonics of detonation.

The knock sensor bar on the 964 was to enable the detection of individual cylinder knock. The ECU looks at the knock sensor output between engine power pulse markers (EPP's), just before TDC compression, and just after TDC firing. The knock sensor bar allows each cylinder to be detected more or less equally.

If you have only one sensor it will have to go on the case, near the center. The frequency of knock is 1000's of Hertz. The frequency of mechanical noise is 100's of Hertz, so don't worry about mechanical noise too much. You will retard all the cylinders if one starts to knock. The knock sensors are "tuned" to some extent based on the number of cylinders. How do you know what sensor to use? I concluded that, with only a single knock sensor, the best place to mount it was in my toolbox.

My sensors were part of the J&S Safeguard system I have. I just haven't used it yet. I believe knock is around 5500-6200Hz; nothing else in the engine is close to that frequency. I am not a detonation expert by any means, BTW.