Tube heaters you want all my spaniels have them in there kennels you get them in 1ft 2ft 3ft I have the 2ft ones they dont get hot hot but take chill of and are cost effective. Yes dogs don't need heat but when my dogs are working 6 days a week on land there's no dought in my mind a dog with a nice warm bed will perform better than one which is cold my labradors are in open front kennels with insulated boxes in them a ft off floor they dont have heat but I feed them warm barf at night to get them through.