I'm currently using Google Sketchup
I know it's not ideal because it doesn't do actual solids and rounded edges are a problem but I've learned to use it (and the STL converter). I've successfully made 3D models with it that I had printed. It works OK for stuff like that.
I'm trying my hand at inventor fusion at the moment too.. But it's proving a bit more difficult (I have no experience at 3D design). It's a shame there's no standard format for these. STL is fairly standard in 3D printing but most programs I know can only export it, not import.
I thought your sketch was mainly a sketch of the dimensions, as I said in the PM recantha2 sent me some but the dimensions of the camera module itself (radius of the round thing at the top), distance of the screw holes to the board edge etc are things I don't know exactly yet. The last time I had something printed I was surprised how precise it turned out (as in tenth of millimetres of accuracy) so it's important to get it accurate.
Having said that, I'll probably wait with ordering my case until I've got the board myself, the last time I designed an enclosure for an LCD it turned out the actual part was several millimetres off spec compared to its official data sheet so I was glad I waited until I got the actual part.
I'm also thinking of making a separate 'slot' for the camera module itself and for its board, fairly close together of course but just a little ridge that will hold the module and stop it rattling against the board.