This reminds me of the ancient ice ponds that made ice thousands of years ago in Persia. I read somewhere that they were able to make ice through a combination of radiative and evaporative cooling at night temperatures around the same as you experienced, about 5C.
This doesn't exist and can't even in principle because it would be a fundamental violation of thermodynamics. Basically, it would be a Maxwell's demon for radiation that would allow you to arbitrarily reduce entropy.
You'd be slowing the flow of heat into your reservoir just as much as you'd be slowing down the loss of that heat later though.
You might still get some benefit in preventing freezing at the expense of needed a larger area to get the same amount of heat flow in to your water system though.
https://en.m.wikipedia.org/wiki/One-way_mirror
They are not perfect (intensity & wavelength limitations), but they exist.
See also mirages and total internal reflection.