Get a rubbermaid type bin. Put your carboy inside and fill up the bin with water until the carboy starts to float, then take out a little water so it won't float. Put an aquarium heater in the water, making sure it is submerged enough. Leave it out there and keep checking it for a couple days to see if it can hold the temperature you need, adjust the aquarium thermostat as need. As a wort ferments, extra heat will be thrown off so if you can almost hold your desired temperature, you'll be fine. If you need to, you can wrap the bin with an old sleeping bag or comforter, you could even cut a hole in the lid for the carboy top to stick out of so you could wrap the top too.
I use just the bin and the aquarium heater on the lowest setting in my basement and maintain ~62F in an unheated basement dipping into the high 40sF some nights.