Location is important, with a free-standing pool wind chill can lead to fluctuating temperatures, so it needs to be sheltered. If you can get bubble wrap or greenhouse fleece (or whatever) contained in a false liner between the wood and the main liner that will help, but the right microhabitat will make this un-necessary in anything other than the most extreme conditions. I've done really well with large wooden half barrels, although I tend to put exotics in them and bring them in or give them away for Winter. The larger you can go, the more stable the water temperature and chemical parameters.
Any alternative if you're any good with wood is to get a large water storage tank such as might be used in agriculture and then clad it, probably significantly cheaper as well. I had one liberated from a garden centre that had been used for displaying aquatic plants.