How to improvise cloches for balcony pots

[Polyethyl]

My carrot and onion seeds in my balcony pots were killed by the beast from the east.
I have replanted more seeds(carrots, peas and spring onions), but more cold weather is forecast. I live in London, so hopefully it won't get too cold again.

But if it does turn freezing again how can I improvise cloches. I have a fair bit of polycotton left over from making a world book day costume. Would it be ok to wrap my pots in polycotton?

I use cling film, also cut plastic bottles in half which you can then fit over the seedlings. Can you move them away from the railings.