I also recently completed an ebay temp controller, but also made a thermowell out of a corny dip tube. Cut the tube in half, crimped one end with a hammer and used a vise to crimp tight, followed by rolling the steel over three times with pliers. This seals the dip tube up without the need for glue or soldering. I don't think I would have done the thermowell with glue/solder.
Now with the ebay temp controller and thermowell, reading from the center of the carboy has provided much better temp control than my Johnson A419 with the probe stuck to the side of the carboy. I think it is also better because in my chambers, the fan blows air that seems to trick the surface temp controller even with a medium covering the actual probe.