While cats do like to sleep on human furniture such as beds, sofas and stuffed chairs, they like it better when a cat-sized mat, blanket or even an item of your clothing is on those furniture items. Cat beds, mats and blankets add extra comfort, and warmth, depending upon what material the mat or blanket is made from, and how warm or cold it is in the house.
Cat Beds: Heated, Luxury & Outdoor Cat Beds | Petco
Also, you might want to see about looking at garage sales & flea markets (those type of venues) for a good rated sleeping bag. If you can't find a good sleeping bag, then try adding more blankets/towels to the two you already have. This will provide more nesting material for the cats to burrow into so they can then snuggle up with each other to keep themselves warm.
Heated Cat Beds, Heating Pads & Outdoor Heated Cat Beds | Petco
Our 3 outside cats stay in a detached garage. There is a pet door that they use to get inside. We use a 250 watt infrared heat lamp wired safely and suspended from the rafters in the garage. The heat lamp draws 2 amps of current, and is much cheaper than any other type of electric heater, as it only draws 250 watts, about the same as your television. This is the type of lamp that farmers use to keep baby animals, like chicks, baby pigs, baby goats, etc. warm in the winter time, and have done this for over 50 years. The lamp heat is infrared, so it is a radiant heat, and is a slow, constant heat when they lie directly under it. These type of lamps are used in barns across the country, and have been for decades. They have a safety grill on them so if they fell, the lamp would not directly touch anything. We also made a bed for them out of a cardboard box, with the top covered, and insulated it with old coats, blankets, etc. The heat lamp is suspended about 2 feet above the top of the box. The cats like to lay on top of the box, under the lamp, and they keep perfectly warm. If you use a heat lamp, you need to get the type they call the Brooder Heat Lamp, for baby chicks. This has the ceramic lamp base, and can take the heat from the bulb. Do not use a trouble light base, since these are usually plastic lamp bases, get the ceramic. We have done this for decades with animals outside, and I don't ever recall any safety issues that any one had from these. I suppose you might hear a story of a barn burning down from a heat lamp, but for every one that this happened, there were thousands of instances where this worked fine. Again, this is for putting in an enclosed area, like a garage, or barn. Here are some examples......
Heated Cat Beds and Pads | CozyWinters