I think for winter protection you'd do better with clear plastic from the hardware store. That's what I always used. The perforated/slitted type will get too cold in really cold weather at night. You're trying to do more than just keep off frost.

There are a few options and a few things you need to decide your goal with the materials. I'm not certain that you are in the low tunnel realm but it really doesn't matter. The low tunnels I've seen are typically for single rows of undetermined (extended) length. For a 4' wide row you'll need .5*4*3.14=6.5' of width just to reach ground on both sides. You should probably add a foot on each side to cover so realistically you need an 8' wide sheet of film.

The UV light degradation of plastics will occur during Spring and Fall seasons too. I've tried to save a few $ on the construction 6 mil plastic and found that it begins to break apart after just a few months of exposure. If you want to reuse the film a UV protected greenhouse film like Tuflite is a good investment. With proper storage you should get many years of use from it. So thickness of the film is not near as important as the type you purchase.

A problem with unpainted PVC is that it emits gasses that will deteriorate the film where it touches.

If you close these covered raised beds tightly expect heat to build up on sunny days, enough to cook plants within. So a few slits to let out excess heat may be helpful. For actual low tunnel cover that comes in rows one friend drills holes through the solid film rolls before laying over crop rows.

Keep in mind that even a 6 mil film is usually only going to buy ?? weeks of season extension depending on weather and your crop. Most growers use them for frost protection.

One thing you might want to consider for your cover is a heavier weight Row Cover fabric like Reemay or Agribon. I've even used a more durable Dupont material called Typar that lets little light thru to the plants but can allow for minimal air exchange and can be reused many years. The thicker of these fabrics can give you up to 6F degree protection for a short time.

Check out the brand names mentioned on the web for suppliers. Nolts in Leola, PA handles a variety of products but you will want a local supplier since shipping expenses can be unbelievable.