Hi Brian, instead of trying to construct a cap directly in place on that 3D curve, it is a lot easier to create a more simple capped end and use a boolean to cut it and that keeps everything as a solid.

So for example, instead of creating a wavy hole at the end, construct your object to come out to a simple flat cap like this:



Now to get your curved end, draw a curve in the side view:



And then use Boolean difference to cut the solid with the curve. Here I have dragged the piece apart slightly:



Just delete the piece you don't need.

Often times it can be a lot easier to build a more simple version of your shape that is extended somewhat, then cut it back to get your final result. It can be a lot more difficult to try and work directly off of a more complex boundary hole.

There is another approach you could use - you happen to already have 2 edges in that opening, so you could select both of them and then use Construct / Loft to make a surface through them. But that surface will be collapsed down to a single point at the top and bottom, it will tend to cause problems later on with fillets and such. The simple cut that I showed above is far more fillet friendly.

Hope this helps!

- Michael