I feel like I should be able to figure this out but I'll be damned ...

So, looking at this:



I run into this situation a lot and I just don't get how to take an open end of something, could be any shape, and just lay a new surface over it to cap the shape. I've tried booleans, blending, etc. There's something obvious here, I'm sure, but it's eluding me.

Help? :)