For federal agencies, the seat pad is now standard. There are no public studies of its effectiveness. Regarding how the U.S. government attempts to detect polygraph countermeasures, see:

https://antipolygraph.org/blog/2018/06/09/ncca-polygraph-cou...

How is that even supposed to work?

A TV viewer thinks "How about butt clenching?", then one of the characters in the TV show articulates this thought and other gives a response. What actually the response is usually does not matter, because TV viewer already felt some connection with the show when that question was asked.

https://www.youtube.com/watch?v=x8w95xIdH4o

By making you believe that it works, like every other aspect of the process.

Then they're going to need to go with an electrical sensor of some sort. Some kind of EBG that could at least plausibly detect it.

Something that's kind of rough and uncomfortable would make the most sense, especially if you had to sit in some exact place for several hours. Discomfort would add to the stress.