Standard oven thermometer should do, just make sure the tip/needle is inside and scale/display is outside the box so you do not need to open it. I have two thermometers with thermistor on 1 m long cord and I plan to use them, when I have spare timine.
The reason behind white roofs is safety regulation, I guess. I asked local police about chrome roof and the answer was i would get pulled over for blinding other drivers.
To CrippleRooster: actually, as far as I know, better than Land Rover white roof was their tropical roof. Essentially white metal roof about one inch above normal roof. It reflects sun radiation and the one-inch air gap prevents heat to be conducted to the car. Images can be found
on Google, one example from ipocars.com:
If you have spare white coroplast sheet, attach it to the roof rails and call it good enough.