A box, big enough for some nice polar fleece blanket and 3 cats, not too much bigger than that would work. Seal up the box and cut an entrance hole and put the blanket in there to make a little "cave'. They should all crawl in there and snuggle to keep warm.
I just always worry about heaters...any portable heater. Fire issues.
Put the box in the kennel with a litterbox and food/water. You'll have to find a way to keep the water from freezing though.