Not sure how effective heaters and even insulation are when you consider most kennel boxes have a big open doorway letting all the heat out
Having a good insulated box with 100% no draft is what I have always been told is needed. Damp and draft free.
I would consider doubling the dogs up to keep warm together if they get on ok.
A lot of lads I know have gone over to more like indoor kennels, where the dogs are not as exposed to the elements. Inside purpose made buildings with drainage etc but normally UPVC doors and Windows so the temp inside never gets down as low as outside anyway.
I genuinely think it’s the damp that gets to old dogs rather than the cold.
I was up a Yorkshire hill farmers place last week, they had 3 collies all live outside in a plywood box on a chain, no insulation, just a box small enough to curl up inside. Everyone of them had a big thick coat from living like that. Out worked on the sheep everyday.
One of the better outdoor kennels I seen the dogs had to walk through like a small walkway to get to the insulated sleeping box. So the sleeping box door didn’t just open to the outside into the cold. Was like a middle chamber to pass through first.
I know one of the lurcher lads insulated his sleeping boxes only, and out one of those electronic thermometers inside to check the temps. Said with the dogs body heat only it was 16C, he said he was stood in his own house which was 12C, worrying about if the dogs were warm enough. Warmer than in his house!!!

