girasole, I would most definitely avoid polycarbonate sheeting.
We have two cold frames that are 'glazed' with polycarbonate sheeting. The are dreadful.
The polycarbonate sheeting seems to shrink, and come away from the frame.
On a windy day, the polycarbonate sheets can be blown in any garden in the village!!
I would go for horticultural glass. It's heavier, and does a much better job.
valmarg