+1 on the Goldenrod. I've had one running in a steel cabinet in an unheated building and another in the safe indoors for the last 8 years - nothing in it has rusted. Condensation is a big problem with unheated storage. Since water will condense on colder than ambient surfaces, such as a rifle in a cabinet as the air temp goes up in the morning, the Goldenrod keeps the contents of the cabinet slightly above the air temperature, preventing that condensate from forming.
Dessicant will work, too, but it can only remove a finite amount of water from the air until it stops working, and needs recharging or replacement. This works better in an air-tight container, where no new damp air is coming in.
A Goldenrod heater can just be placed on the bottom of the cabinet. You'd get the same effect from putting a light bulb in there, but it won't last, and will need to be checked. If it burned out during a humid week with large day and night temperature swings, condensation and rust can happen very quickly.
Found that out the hard way.
However, if I hadn't rusted those dies, I never would have discovered how well
Evapo-rust works.