I've seen all sorts of winter covers used. Some people pay to have their boat shrink wrapped. Other's buy a custom winter cover with plastic windows from a canvas worker.
We used to scavange used pine planks form the marina yard to build our frame. We'd frame from the bow to the stern, and make sure the frame had a good pitch to prevent snow buildup. We'd attach the frame to stantions and pulpit with plastic cable ties.
We'd buy regular clear polyethene plastic that comes by the foot/ meter at your local building yard. White or coloured plastic won't let in light and you will be living in a cave. And duck tape. LOTS of duck tape. And a good staple gun and staples.
We'd duct tape the plastic to the frame and staple over the duct tape. (If you don't use duct tape the plastic will tear in the wind.) You need to get a good stretch so that the cover won't flap and make a racket in the wind. Also a nice tight cover won't rip.
It was always a fair bit of work and inevitably we'd be putting up our winter cover as the first snow flew... but it was cheap and warm.
To prevent condensation (now there's a topic...) we'd cover our deck with insulation wrapped in garbage bags. That was more or less successful...
Oh now I'm so glad we went south! It's 31 degrees here today and we're just taking down our sun awning