Night time temperatures where I live are getting down to around 10 degrees which is heading towards being too cold for piggies particularly youngsters who are only used to being inside. Ideal temperatures for piggies is around 18 degrees. Keeping piggies fully outside (ie not in a shed), takes a lot of work. A thermal and waterproof hutch cover will not be enough to keep them warm throughout the winter.
My two piggies live in a hutch in a shed. It keeps the temperature about five degrees warmer than the outside temperature. It’s primary use is to keep wind and rain out but even with it being marginally warmer than directly outside it takes A LOT of effort to keep them warm in winter. Even in the shed a thermal cover isn’t enough to keep them warm. They have two blankets over the front, then the thermal cover and a duvet thrown over all of it. Inside the hutch they have loads of hay, hides and boxes filled with more hay and four microwaveable snugglesafes (between the two piggies) for them to rest on. These measures keep them at around 10-12 inside the hutch (ie the blankets etc trap the heat) when it is around 0 outside, so you can see that even these measures only provide about 5 degrees of extra warmth above the shed. So fully outside you’d need to do more.