Cadence irregular on Zwift

I think it can be anything. If you wave your hand next to the sensor it registers movement..

The higher the permittivity (measure of the electric polarizability of a material) the better. So any metal should be fine, no magnetic properties necessary

That metal plate is not straight, is it ? Otherwise it hits the bike frame when the crank arm points backwards.

Mine is straight, I ride a MTB and I made the extension short enough. However, you can bend the end, such that it is parallel to the frame. Just as if you'd use a spoon as an extension. The extension Tacx sends you is straight by the way.

I have a standard road bike frame. The chainstays widen at the rear wheel hub. The metal plate needs to be bent in my case. But I think I could get away better if I stick a magnet (e.g., a disk-shaped neodymium) -- or, even better, ha ! several layers of Aluminium foil pressed and shaped & glued over the shoe -- on my left shoe, somewhere towards the heel, as that part already enters the Neo2T's sensitivity zone when I pedal.

As I wrote above, magnets won't help you. 

But will magnets HURT you??  I just got a Neo 2T yesterday and I also have the cadence problem.  The bike is my trainer mule... an old Trek 2300 Carbon with Ultegra 8 speed shift gear and 175mm Ultegra cranks.  Cadence is constantly jumping up and down about 10-20 RPM.  I have installed the latest firmware (.34??) and it didn't make a difference.  I see that Tacx will provide an extension plate for the left crank arm so I guess I'll try to get that next week.  One question:  I still have a magnet on the end of the left crank for using my other dumb trainer with a Garmin speed/cadence sensor mounted to the chainstay.  Is it possible that magnet is interfering with the Neo 2T cadence sensor?  I know, I can pull it of and see if it helps.  Just wondering if anyone else here may have tried that.

Edit: this is on Trainerroad not Zwyft, on Bluetooth, but the problem is in the signal so should be the same for all platforms.

Well, removing the magnet made no difference.  So I found a piece of scrap aluminum I had lying around and duct taped it to the crank arm. Cadence is now rock solid perfect.  This is a little annoying for such a pricey piece of gear but since this bike never leaves the trainer it isn't really an issue for me.

Looking good! 

sorry but this is a crap, 1200 eur trainer should work as it advertise, not with some improvisation