My 1940 also has but one light. I use this small portable LED light when driving at night. Is the kind of warning light that might be hung on a farm/work trailer. Red LED is very bright, will certainly catch the attention of a car behind you. (maybe too bright; had it in flashing mode and club member driving behind me told me it was obnoxious!)
It's hook fits perfectly over the '40 bumper, so should also on a '39. Magnet on back of light grabs the bumper so doesn't bounce around. Batteries last me a whole season.
https://go.harborfreight.com/sku/64115/