It was 22F in the coop this morning.
When I retrieved my eggs, they were 42F.
Heated boxes working as expected.
https://www.backyardchickens.com/a/heated-nesting-boxes-help-stop-frozen-eggs
My seedling mats are keeping the nests, and eggs, 19F degrees above ambient nest temperatures.
The mats are fairly inexpensive as compared to other heat sources.
This should (hoping) help keep eggs unfrozen to temps as low as 10F.