Hi Stuart - it looks like both Jamie and Burr have you covered with some steps to get what you want above?

That method of trimming out a piece and then either shelling or extruding the trimmed fragment is probably the best way to do this.

Also check out these previous posts from some other illustrations on this method:

http://moi3d.com/forum/index.php?webtag=MOI&msg=3024.4
http://moi3d.com/forum/index.php?webtag=MOI&msg=3318.1

- Michael

As a side note, I did this too just for fun.

Select pipe.
Do a trim, selecting the panel curve as the cutting object (keeping all parts).
Delete extra panel curve at bottom.
Do an Inset on the panel (using separate height).
Boolean pieces together.

Worked fine :)

Inset command is good!