Google Sketchup is supposed to be easy to use for 3D design of that very sort of thing, I've messed around with it a little, except I really had no use for it and just wanted to compare it to other programs. It might work out pretty well for "real-world" designing such as that.
I mean about making the plans with actual measurements not just loose 3D design "Sketch"...
), I've found some other stuff.If you can get ahold of it autodesk inventor is a pretty straightforward program. Very user friendly, especially if you have used Autocad before. I have never used Solidworks before, but it is supposed to be very nice also. As of right now CATIA and Solidworks are the industry standard for 3d cad work.
btw why are you using mdf? is there any reasoning behind that? I am curious. I think Plexi would be a much better option.....
I'm trying to make this case as silent as possible 
Is that the only view you have? Considering that is the side, a front and top view would make it easier to understand, but besides that, it is a pretty good drawing.
What did you end up using to do it?