Hi Thomas - Planar won't work in this case because the top edges are not flat. Looking at them from the front view, they have a curve to them:




But you can use a different method to create a solid there - select the 2 top edges and then run Construct / Loft to build a surface between them. Then select that surface and the rest and use Edit/Join to make it into a solid.

However, I don't think that you're going to have any luck shelling this in MoI - MoI's sheller is pretty sensitive. There are 2 situations here that it won't like - the tight bend near one end, and also the way the surface collapses down to one flattened edge at the bottom.

Shell won't be able to handle this situation - for this one you'll have to manually build the inside or outside surface.

Check out here: http://moi3d.com/forum/index.php?webtag=MOI&msg=784.4 for a recent discussion on a similar model.

- Michael