I had the very same problem installing the temperature sensor for the VDO 
CHT gauge in the spark plug hole. I "fixed" it the following way:
I cut off the metal ring attached to the thermocouple, leaving a small 
piece of metal around the thermocouple behind, which has the same width 
like the gap between two cooling fins. I squeezed the whole thing between 
the two cooling fins closest to the exhaust manifold on top of the 
cylinder head. Still close to the spark plug hole. I drilled a small hole 
into the engine tin to get the two wires into the engine compartment.
Before, I had the sensor mounted replacing the spark plug washer and the 
temperature readings now are still the same. BTW I did that for all 4 
cylinders. 
The cables for the 4 CHT sensors run underneath my bus to the front to a 
double pole/single pull rotary switch mounted right next to the CHT gauge. 
The switch is connected to one VDO CHT gauge. In such a way I can scan
through all 4 cylinders simply by rotating the switch one position.
If there is a better way to mount those sensors I definitely would like to 
hear about it.

Stefan
'76 Camper Bulli (LILA)
'78 Rabbit Diesel