GIGO: a high-tech expression that stands for "Garbage In, Garbage Out"
If you use a "cheap" piece of furniture from Target, you'll have a "cheap" humidor when you're done ... but perhaps it will not be an inexpensive one .... far better to pick up a piece of old furniture at an estate sale, a garage sale, a flea market or wherever ... finish is unimportant ... integrity of the item IS ... after you use the spanish cedar to line it, you can always refinish it ...
You should know that unless it's a true antique, it will almost certainly be a high-grade wood veneer over a MDF or HDF board and you should use great care to not sand through the veneer.
There's not much danger in the wood warping from the humidity of a humidor ... after all, lots of people, many of them in this forum, live in climates where the ambient humidity is frequently higher for a sustained period than inside their humidor and their furniture doesn't warp ...
Go back in the pics board and find the pics that one of our guys posted of his homemade humidor ... I can't remember who it was now, but it was an exceptionally nice job .... late spring or early summer I think ...