Hi Duke - the only thing to keep in mind is if you want an object with a uniform "wall thickness", then usually making a copy of an object won't produce that, you need to use Offset or Shell generally to have a result with a uniform same thickness everywhere.

But if you don't need uniform thickness, then something like what you are describing should be fine.

It looks like the problem with your model for filleting is that you've got a duplicated surface in there, I mean there is one surface that is actually 2 surfaces stacked up on top of each other, it is one of the interior ones. If you use Edit/Separate to break this model into individual surfaces, and then drag the interior pieces you'll find the one that is duplicated because it will leave its duplicate behind when you drag. And when you select it you'll get a kind of "speckly" looking selection which happens when there are 2 objects exactly coincident with each other, with one selected and one not.


Once you eliminate that duplicated surface and rejoin the other pieces up then you'll have a solid and those edges that you were looking at should Fillet fine then:




You'll probably have some problems filleting the upper outside edges though, with the upper cap surface made up of multiple pieces like this:



I don't know if you'll want to fillet that upper outside edge, but you'll probably need to make that one broader surface with a hole trimmed in the middle of it, to make that filletable rather than have it made up of 4 fragments.

- Michael