I have used isopropanol (rubbing) alcohol for this purpose. It is cheaper than cheap vodka and is less likely to get you selected for a sobriety test.
The net says you need a 45% mixture of ethanol and water to get to -20F. You would need 180 proof alcohol + 50% of the mixture being your added alcohol to get to this value.
Isopropanol needs to be 70% to get similar protection.
Note that the fumes from either of these additives at these concentrations can be explosive and unlike your gas tank, the windshield washer reservoir was not designed to be safe when filled with such gases.
In some states windshield washer fluid with antifreeze is not available in stores, probably due to concern over the toxicity of antifreeze. I'll bet that AR is filling the reservoir with this dangerous fluid in order to comply with state laws. Since the nearest grocery story to my house in Bridgeport, Ca. is in Nevada, I stop at a auto parts store while I am there and get some -20F or -40F rated "good stuff". However, I have noticed that the -40F rated wiper fluid is not very good at cleaning goo (like bug splats) off of the windshield.
Some cars have heater windshield washer sprayers, but I don't think they heat the reservoir.