David wrote:

I would like to be able to measure the ambient temperature without
carrying a bulky device.
Favourite option would be an attachment to a mobile phone.

Maybe your phone already has a temperature sensor?

Some of the on-board barometers include one, and most of the batteries
will have one (though that probably will be a few degrees higher than
ambient and won't react quickly to changes)

Install the CPU-Z app and see what it can tell you ...