Mare motels are a "covering" that is very popular out west.
It is a pipe rail confinement with a roof and no sides and can be built in various sizes.
I lived in SoCal five years, where mare motels also abound, but I never did find out how those types of open shelters got that name
Ditto shut the water off to the waterer and put a bucket in there.
Your horse should be able to stay warm enough provided you don't end up with a sideways wind that would drive rain right thru the shelter.
I would have a waterproof blanket on hand in case she starts to shiver. I am not one of those people that wants to see my horse burn calories and stress by shivering to stay warm.
Once she warms up, or it comes daylight, take the blanket off.
This weather is a bigger eye-popper than last year. Our low for January 10-11 was 63 - that breaks records since Nashville, TN started keeping weather records in 1890. By the middle of next week those night time temps are going plummet into the high 20's - low 30's.
If this isn't colic preparedness weather, I don't know what is. Folks that can afford to buy a tube of Banamine and keep it in the refrigerator, need to