If they’re in a dedicated room, could you put an electric heater with thermostat setting in there? I’ve used one in the front room for both guests and pets at various times (on a table, and they do shut off if knocked over or bumped). And I do understand... we’ve had our grumpy old man gulf coast box since 1995 ( I inherited him).
I will probably have to do something like that and probably a small water heater too. I just need to keep the temps 72 degrees or higher. I remember your little gulf boxie.... I always get a kick out of seeing their eyes when they see a plate of fruits and vegetables.