I never keep water in a coop. Add's moisture. Moisture leads to frostbite on combs. Plus itjust makes a mess. I let me birds out early. Then the feed and drink. Never either in the coopPlexiglas isn't cheap either!! At least around here a 4x8 sheet is over $120.
And yes, horse manure would probably be a lot less smelly than fermenting cow manure.
Are you hoping that if the water freezes overnight the greenhouse will thaw it? I'm wondering if there would be any value in being able to cover it at night to keep some heat in. I assume the birds want a morning drink when they get off the roost.
Bruce