I'd knock up a box in tongue and groove and make a wooden divider using a jigsaw. Sand it, stain it and varnish if you really want, and it won't stand out against quality stuff.
If you get an old packing case the correct size, or knock something up from pallet wood, you can laminate it with 3mm ply and stain etc. That's dirt cheap and looks OK too.
If you draw up a design, you could get the sides etc cut of ply at a local builders merchants and screw them together yourself.
I've made a few units in similar fashion.