Welcome RONZOG!
A refrigerator will have the appropriate temperatures to maintain dormancy without the need for a heater or anything else inside of it ( as long as it is not one that can be set on a setting where it stays above 45F (temperature should be between 32F and 45F).
I actually "wintered" trees in refrigerators for about 13 years when I lived in Orlando, so I know that it works, as long as (of course) one does not forget to water the trees. Make sure to keep the substrate evenly moist at all times.
In your location, you have the added benefit off having a normal fall at the correct time, so after your deciduous trees lose their leaves, you can just put them in the refrigerator. Or you can wait until later when temperatures are forecast to get low enough that it may do damage to the trees in question.
Of course, there are ways to over-winter temperate bonsai outside in cold climates utilizing a cold green house, minimally heated shed, cold frame or munched in outside with snow for insulation. Which species are you wanting to over-winter?
Oh, also, it is suggested that you include your approximate location in your profile so members can give you the best advice going forward, with your location taken into consideration (though I see that you mentioned it in this post).