Summary for those one the run: “umbrella”.

Long story: if you are walking somewhere at night and expect a drone operator with thermal vision to try finding you, have the biggest umbrella that you can get.

Since it’s not in contact with your body, it will be at the temperature of the environment. It will catch the heat radiated by your body and re-emit only a tiny fraction, shedding most heat to air via convection.

Be aware. Metallic umbrellas may (or may not) make you looke like a black hole walking around. This may not be desirable.

If your umbrella is wet, that’s even better.

Note: thermal vision and NIR (near infrared, requires a NIR light source, used by some security cameras) are not the same thing - for NIR, principles of visual camouflage apply. Also, not all night vision is thermal, some night vision devices use light amplification.