Rad fan sensor ideas

I put the radiator in the American this weekend with it's electric fan in place, Tight fit!.
I have a relay wired up in the relay center to power it.
Now I'm trying to figure out how to trigger the relay.
No place in the engine to add a sensor that I can think of.
My options are, to use the relay I have in place and trigger it with a switch. Then I would add one of the adjustable t-stat systems as a main unit and use the switch as backup.
The other thought is to get a piece of pipe and put a bung in it and use a threaded snsor to trip the relay.
   Anyone have any thoughts on any of this?
Anyone know a good source for a threaded fan switch?
Autotronics makes a nice one that has a ground wire that would be needed with the radiator hose mount, but the sensor itself is $80!

Thanks, guys

Mark Price