Walls and Ceilings - Paneling/white board vs. drywall

I recently tore down a non load bearing wall in my basement. The ductwork is on either side of where the wall was. I left as much of the old 70's paneling I could (I'll deal with that later), but I had to tear down the paneling that framed in the duct work since the wall is no longer there to support it. I'm sure drywall would be the right way to finish it, but I'm looking for the cheapest/easiest way to just cover it up at this point. There is about a 1/2 inch height difference on one side and a pole in the middle that attaches to the beam (which also needs to be framed in now). Is it possible to use plywood, white board, or anything else other than drywall? Due to the height difference, I think it needs to be more flexible than drywall. Also, it will need to be pretty strong since I won't be able to nail/screw it in the center...only on the edges. The dimensions are 4 x 30 ft. Any advice would be much appreciated!!!

With a width of 4' it would be best to frame out the entire length with 2x lumber, adding support in the middle to support whatever medium you choose to put there. In that manner you can correct the 1/2" deviation and make it flat. I would use sheetrock. Plywood will not flex, and masonite will sag and is quite difficult to nail into without problems.