ummmmmm, well that's quite a situation that you got yourself into there. Since this thing is going to be painted, it doesn't matter how much filler you end up using, so it's possible to 'add wood' to your project. Somethign that you really cant do when making furniture and hide it well.
You can take one of your sides and use that as a pattern on another piece of mdf, that will give you a match for the curve that you have there. From there, you can redesign the marquee and top edge. Once you get those pieces cut out, just attach a 4" wide board that you'll be able to glue and screw the new and old pieces to. Use a 2" overlap to handle any stresses that are generated. Be sure to use glue on all mating surfaces to ensure the strongest possible bond. You can also use dowels or biscuits between the new and old side pieces for additional strength.
Once you have it together, you can just use wood filler to fill in the gap and to cover up any screw heads. Once sanded smooth and painted you shouldn't even notice your previous error. With the addition of the top and back panels, the whole cabinet should be rigid.
When going thnrough this process, keep in mind where any additional boards will be going for supports. These may have to be modified due to the above rework.
Good Luck!
Woodworking is practice based. If you're not making mistakes, you're not learning.
