Hi Randy. I've always thought about doing one of those, but never got around to it. So far, I've trusted myself and my wife to notice the needle going high and me looking under the hood enough. Have you thought about running one of those common float switches to the normally closed side of a relay that runs the dash light? Both sides of the relay should be on KP-II for power. As long as there is coolant the relay will be energized by the float switch and keep the dash light out. It would work, but would normally draw a little power. I see small 75 mm (3" total length, maybe 2-3/8" stem height) white poly float switches similar to yours pictured on Aliexpress at the moment for under $3 counting shipping https://www.aliexpress.com/item/1005004587331649.html?. It's not clear if those float switches are closed at top or bottom of travel, but at that price it might be worth the experiment. Cheap 12V solid state relays that you can solder to and only draw an amp or two can also be found there for a couple of bucks. Used Volvo reservoirs out of a 760/960 from a yard would seem another option.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now