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I'd suggest a VERY low wattage outdoor bedbox / cathouse made up to keep them warm & out of the elements without leaking loverly heat out of the house (& as for tomcats, leaks indoors).
These used to be really hard to find specialist stuff, now more mainstream.
ok they draw power, however less than the loss from a leaky hole in your door one presumes?
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Flexiguard 33: 33cm x 22cm (8.5" x 13")
Flexiguard 44: 44cm x 33cm (13" x 17.5")
Flexiguard 55: 55cm x 44cm (17.5" x 22")
cover it over with a soft blanket (for washing regularly / vet bedding blanket) with a block of spare insulation to deflect heat upward (obviously) ..make a wee waterproof insulated draughtproof box & hope your ingrate felines deign to use it!
There are heated cat "pods" boxes for outdoor buildings that could be improved for outdoors..
I'm of the type that you get your cat in the evening to allow wildlife to feed (ditto early morning) a cat shouldn't be out all night ..I'd want it's warmth & company as well as knowing it wasn't catfighting & waking folk! so would stick this on a timer, that'll learn em!