Hi Jerry,
The temp probe/thermoswitch for my TR4's elec fan is basically the same as Bill's, a bung welded to the lower radiator pipe and a probe that threads into it.
An alternative to this is a T-connector that goes into the radiator hose, but the hoses on my car aren't that long and make sharp angles. It's basically the same idea, but doesn't require any welding, just cutting a section out of the hose to insert the connector.
Yes, there are sensors that slip under the end of a radiator hose. I'm not wild about the idea, too likely to leak or plow the hose off, or crimp the sensor with the hose clamp, for my tastes.
I'm also not all that thrilled about the type that just slip into the radiator fan or otherwise attach externally. Ambient air temps have to mess around with the accuracy of this type. I'd much rather the sensor detect coolant temperature, as directly as possible.
You can get the varous types of sensors at
www.summitracing.com, either as part of a complete wiring kit or as individual parts.
Cheers!