What is the appropriate way to seal an opening in a pressurized tube around an inserted object?
Many years ago at the beginning of my engineering career we made sealed temperature sensors using silicon diodes bonded onto the ends of Teflon insulated wire. The wires were then fed through two holes drilled through a short length of 0.25" diameter Teflon rod. The rod was the inserted into a standard brass compression fitting using a standard brass compression ring that tightly sealed the Teflon rod and formed it very tightly around the pass through wires. 

  

What you see above is a heater assembly that produced a temperature controlled gas stream in the range of -55C to +125C. The two diode assemblies on the heater were for redundant critical over temperature safety cutouts and the one at the pipe TEE was used to monitor the gas stream temperature for closed loop control of the heater assembly. Thermal mass of of the system was part of maintaining a stable gas stream temperature. 

Note that silicon diodes biased at a constant current of 1mA produce excellent temperature sensors that are linear over the whole operating range of -55to+135C.