Just use horticultural fleece when it is forecast to be at freezing temperatures for long periods. If you want to create a frost protector around palms then you have to construct a box around each of them (just use bamboo canes and chicken wire) and wrap the plastic sheets around that - not on the plants themselves. You should uncover ASAP after temperatures increase.

For Cordylines, they do not like to dry out though also not too wet either (free draining damp!), even in winter, opposite for Palms, the drier the soil the hardier the palm.