Hi Rob - yeah normally you would do as Anis shows there and do an extrusion from the side followed by a trim to produce a cap on that kind of an arrangement.

You can use that method any time the profile is a distinct 2D shape as seen from the side.

Sometimes it can be easier to extrude something like that as a tall block and then use 2 cuts to slice off the ends to produce the angled results, that way everything stays as a solid through every step. See this recent post for an overview of that method:
http://moi3d.com/forum/index.php?webtag=MOI&msg=2614.10

When you have a more irregular hole to fill that does not have a single distinct side profile, then it becomes a much different situation. That's when a NURBS tool called "N-sided patch" can be useful to fill in things, but MoI does not have that particular tool yet. If you need an more wavy surface as the cap, then it usually becomes easier to build the base piece as an extended solid and cut off the end by booleaning it with the surface so that the surface becomes the cap as it cuts the solid.

Let me know if you need any more info on these other methods, but I think the link above should help give the picture.

- Michael