Most of the hardwoods would be quite suitable to use: oak, cherry, poplar, etc. Some of the softer woods include: maple, aspen, apple, and coniferous trees (pines).
However I would try to avoid pine if I could, and cedar at all costs. They contain chemicals (phenols) that can be harmful to the respiratory tract of cornsnakes.
That being said, most of the pine lumber commercially available is kiln dried, and generally doesn't smell like pine any longer. So I'm sure that it would be safe enough to use for a snake hide. I think you just want to steer clear of the pine shavings, which are essentially raw wood. And consequently most of the snake/rodent hides I see for sale are made out of pine.
I see nothing wrong with paint or stains so long as they have been allowed to cure for a couple of weeks prior to using them, if you could manage to wait that long. There are a few people who house their animals in homemade vivs that were custom stained and/or painted with no ill effects. So who's to say.
Lots of corns in the wild live near pine thickets and human trash, doesn't seem to be affecting their numbers noticeably.
Let us know what style and wood you've decided on, I've never ventured into making my own hides, to be honest. It sounds interesting.