We put a little bit of dry charcoal in ours and use a 7 1/2 watt bulb when we're not opening it much and that has worked for years..no moisture problems. If the frig or freezer seals up tight a small bulb will build up heat but to keep them dry it's more about humidity..buy a cheap humidity gauge and that will tell the story better than temperature will. If you have moisture in your rods and want to use something cheap you can use a small electric counter top oven.
Hope this helps.