I think due be surprised how much insulation does help although i see your point.
Maybe instead of perspex get some clear strong plastic bags and using drawing pins secure it to doors, this will allow flexibility around bottles and easy to cut holes for aeration.
then on the warmer days you can fold it up so front is completely open.
On my double tier my dad basically put thick clear plascti bags (not the shopping bags) but other bags i use for work down each end, then got tongue and groove wood, and put that over the top so 2 layers of wood with plastic in-between, and then the tarp hangs down the back and nailed in and over hang at front to completely cover front of a night, he fitted a frame across the top at front so when rains i can pull frame out, roll the tarp up and over it and it creates a canopy thus keeping them dry.
Provided with cozies, hay, and the snuggle safe heat pads should be cozy - obviously on the terrible days lie just keep them covered