The sensor grenade gave me the idea that a small white plastic tablet tub upturned would do the job.
One of our cats has continual mediaction for a thyroid problem so Ill grab the next empty one.
Maybe with some foil wrapped over it to reflect the sun and leave the bottom open.
Ive found that even a white box in the sun doesnt reflect enough and warms too much from sunlight.
The box hasnt enough ventilation either.
I got around 22C yesterday in the sun when I know it was around 11C.
Gordon