If you want super-cheap, the Visonic contact sensors include temperature, are reasonably small, and often go on sale for $10 at gearbest.com. If you switch the device handler to “SmartSense Temp/Humidity Sensor” they will still operate locally but show temperature as the primary property. I haven’t done any testing / comparison to verify their measurement accuracy; others have suggested it’s not very good, but I only use them to turn ceiling fans on and off, so they’re good enough for me.
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