How to heat an unheated propagator????

kiwifruit_2

dont know how big your propogator is but i have a cold frame and when i need to get some 'warmth' into it, i fill clean 4L milk bottles with about a quarter cold water and then fill the rest with near boiling water from the kettle-leaving about an inch from top and put the lid on. Then i cover with foil and stand inside the cold frame. Seems to put out enough warmth to do the job....apologies if its just a bit to hill billyish lmao...