I considered that you could have vibration sensor plates under player's feet but I can imagine several ways this "doesn't work":

1) Feet could be stimulated using electrical voltage (low level shocks).

2) Cheaters could put one foot on their knee and the system would only activate vibration when it was near a 90-degree rotation.

3) Cheaters could incorporate a vibration-damping polymer like sorbothane, probably a particularly low durometer to absorb vibrations between shoe insert and floor plate.

I believe the answer is going to have to be establishing a "secure" zone that can't be crossed by anyone without a full x-ray scan of all personal effects and mmWave scanners. If clothing blocks the mmWave scan, people would have to don lighter / more form-fitting clothing while going through the mmWave scanner, send their preferred clothes through the x-ray machine, and then swap into their desired clothes in a secure changing room/bathroom.

The main downside to this is increased cost; I'm not even sure how much this would cost to operate. And for which events would FIDE make this extra cost a requirement? Every FIDE rated event seems completely unreasonable - many of these are small local events with very little budget and lots of 1200 rated players. Perhaps any rated event which includes any of the top-10 players? Is there enough money at that level of chess to fund a requirement like this?

Still some potential for hiding cheating devices in relatively private areas like bathrooms, changing rooms, utility closets, or even "planting" large objects like potted plants/etc with hidden compartments. Most likely I'd imagine the player wouldn't grab these, they'd have someone they trust hide them in the weeks before the event and have a person retrieve these and then drop them in a secure bathroom stall/etc. These would be, for example, identical shoes to the ones they came in with.

Perhaps worth having players go through the scanners again right before they sit down at the table, including in the middle of the match if they take a bathroom break/etc. Maybe that would work, but I'm still concerned about the price -- that would need a separate analysis. How much money is available for each of these matches?

The stakes right now are pretty personal but if nations governments get involved in the cheating for reasons of national pride like they do for the Olympics[0] then I'm not sure anyone would be able to stop the cheating.

Another strategy might be to change the format of the top level of chess to "allow" cheating by giving everyone access to whatever engine they want, powered by identical hardware and watt-limited. So the competition would be "man+machine" vs "man+machine". There's been some chat about this but I'm not sure that matches wouldn't be so insanely even that you'd need 300+ games to build a reasonable confidence interval so that you can even determine which player "won". Currently the TCEC (highest level engine vs. engine championship) uses 22 games per matchup to determine a clear result. Even that would be excessive.

0: https://en.wikipedia.org/wiki/Icarus_(2017_film)

1: https://en.wikipedia.org/wiki/Top_Chess_Engine_Championship#....