Plywood box, light bulb on a rheostat, two wet wrung out hand towels in the box.
Try for 90°F and 90% RH. Sort of like East Texas, but works in dry Colorado.
Chuck
Sounds like the box I made nearly 40 years ago as an experiment. I'm still using it. The full length door hinge is and old leather belt, complete with four buckle hole for venting.
Mine has a build in shelf with a small rubber water dish just under the light bulb. No steel anywhere to rust or scratch.
Tall enough inside to rust a 42" M/L barrel. I try for 80% and 80*. Wet paper towel scraps for extra humidity. Two hours rusting time.
We will be using it in July for my Metal Prep & Rust Bluing Seminar.