It would be painted and being in East Texas it's almost as humid outside the humidor as it is inside it. Lol.I wouldn't introduce metal into a humid environment, You can get SC planks at amazon.
I agree with @ApeSmokes regarding the plexiglass. You can find pieces cheep and put it to good use. However if you are trying to use up your scrap I would look to see if your metal is similar to what boveda uses for their bag holders. The other thing I would say to do is just apply cedar sheets from cigar boxes to the sides of the metal divider you end up using, if you decide to go that route, rather than painting it (aside from powder coating, but if your trying to save money this isn't really cost effective). It also depends on the steel, if its stainless I'd be really hesitant to put it in their with the humidity. I think your best route is either buy a scrap piece of plexi or buy/recycle an old cigar box and cut some pieces out of it.A little bit of SC to make a divider shouldn't be too much $$, but if you want cheap/easy cut a piece of plexiglass. Many folks use it in their winadors without issue.