If the plant is in a pot, probably your best bet is to set the pot near a wall that receives plenty of sun. That makes an excellent heat sink. Also, having even the slightest overhang will stop the temperature from dropping quite so much at night. Instead of clear plastic, bubblewrap makes a wonderful insulation for plants.

I have a suggestion that's regarded as unsightly but effective. Hopefully, you can come up with a way to decorate this scheme, but here goes:

This idea works on the "wall-o-water" principle, but uses water-filled soda-pop 2-liter bottles. Water has a unique type of thermal mass phenomenon that isn't replicated by any other medum, absorbing quite a bit of heat in the sunlight, and slow to lose heat overnight. I'm in zone 7 and where it's the case that we only get a couple of days below freezing in a row and thaws out a couple of other days in a row, here's what's been happening:

The ring of water bottles keeps the stuff it surrounds at or above freezing, even when temperatures dip, overnight, into the 'teens (Farenheit). A day's worth of sun is enough to "recharge" the water temperature to survive another night, and so on.

A double ring of water bottles will survive a slightly longer freezing period, where the outside bottles develop ice inside but the inner ones won't.

i use dark colored buckets of water to absorb day heat to release at night in my 6' plastic "puptent" greenhouse. works well.