Small Surface Sensor for DIY Relay

I’m building a 12v trigger relay for an external subwoofer amp. It will be housed in a 4x4 steel electrical box.

Trying to see if there is a surface temperature sensor that I can stick to it that can monitor the temperature and if it goes over a certain level, that can warn me. Ideally integrated with my Smartthings ecosystem.

More of a “safe vs sorry” sort of thing, probably overkill.

I use Aeotec / SmartThings Multipurpose Sensors on my washing machine and oven and they warm up a lot.

Not quite surface but maybe this has potential: