Thermal Divider

I have a thermometer that reads from 0 to 200°F. I'm trying to measure a
cylinder head on an aircooled engine where my range of interest goes from,
say, 200 to 400°F. The classic way of measuring this temperature is with a
thermocouple mounted on a copper washer underneath the spark plug of the
cylinder that you determine (by trial and error) to be the hottest.

I can think of several ways of measuring a cooler spot on the cylinder that
will probably be in rough proportion to the actual temperature at the plug
seat, but most of them are dependent on the airflow over the cylinder(s)
remaining constant from day to day. With the baffling on the engine being
rather thin and wobbly, I can't count on this airflow being truly constant.

The sensor on my thermometer is a plain old silicon diode that won't
directly take the heat that I'm trying to measure. Anybody got a clever way
of making a thermal divider that won't be subject to the day to day shuffle
of the airflow over the cylinders?

Jim